I am currently running an Eberron game, and my party just came into a nice chunk of money. Some of the players are asking for items I just don't have any idea how to price - I know a +1 weapon and a weapon of Warning are both Uncommon,  but what about a +1 Weapon of Warning?  Or a +1 Weapon of Warning and Returning? I remember stacking enchantments being a thing in 3rd Edition,  but haven't seen it in 5e yet, so I'm a little curious about how these things might work.  Thanks! 

I think it depends on many factors. The first is how prevalent is magic in your world. If you can pick up a magic item at almost any shop then it is going to be cheaper than if you have to trek across mountains and deserts before fighting a powerful creature, just to get a glimpse of a magical item.

Next consider how much money your players have and if you want them to have it. If you feel that you want them to be able to have it then put it within their price range but only just. Something a little bit more powerful could be just out of their price range. Remember, money isn't the only way to get something. Maybe the shop keeper wants them to do something for them which gives you a nice side quest. 

Usefulness of the item is another factor. If you feel a +1 weapon is better than say chainmail then it might cost more. 

I personally feel that stacking enchantments onto a weapon is fine but it will increase the cost exponentially. It would be quite complicated and time consuming to fit multiple magical effects on an item. For example (these aren't actual prices) a +1 weapon may be 250 gold but with another effect it may be 400 and then 600 with another. A good place to start it something similar that you do know the price of. 

In my current campaign gold and magic items flow like water. The party is very powerful but I try and adjust the world and the encounter accordingly. At the end of the day, the most important thing is whether or not it will have an overall positive effect on your campaign. My players lover being able to shrug off damage and then shatter the earth with a single punch but not everyone wants that. If you are running a gritty, gruelling campaign where combat is difficult then magical items should be harder to obtain, which you can do with price. If your players want to be powerful and you feel they won't get bored of that then it is fine making it a little easier to get. 

Magical items are rewards. If they are the only rewards your players care about you need to be careful but if you can find other things then you can try things out a bit more.

I hope you find this helpful.

Xanthar tells you about magic item prices.  On the whole, an item that combines the abilities of multiple different magic items should cost a good deal more than each one individually, as it has more utility.

My own campaign is moderately low magic.  Items that are enchanted are typically hundreds, if not thousands of years old.  The formulae have been lost to the ages.  The race that created them is extinct.  As such, Magic Items of any kind are only for sale at invitation only auctions, and you will need a contact in the nobility to get an invite.  Nobody will really know what the item does.  The auctioneers might lie.  So items go for thousands of gold, sometimes tens of thousands for the cool stuff.  Even simple consumables like potions and scrolls are not available easily.  Expensive and exclusive stores may sell them.

Good luck and have fun.

I disagree.

1) The idea that there won't be a magic item store is foolish.   It breaks the realism for no reason, and does not make the game more fun.   There are lots of ways to restrict stuff without shouting NO, NOTHING.    Make it hard to get and more expensive at each level than what people typically use.   Put in taxes, guilds, etc.      

2) I love "Sane Magical Item Prices"  Link.   If you want magic items to be more expensive or less, I suggest applying a set factor (i.e. x2, x  1/2) etc.   But this is a well thought out relative pricing for everything from PHB, and maybe a bit more, do not remember.

3) If magic items cause problems, or just to screw with the players, I suggest you also adding some ways to lose the items.   Thieves exist.   Cities might confiscate a Wand of Fireballs. "You brought WHAT inside our wooden city?"    Use Disarm against them.   Throw in an "Greater Rust Monster" that works on magic items.    

4) Curse the crap out of them.  Especially with one of those "Hm, maybe worth it...." such as +2 weapon that sings (badly),  ruining surprise.  

The Sane Magical Item Prices thing is really great.  I don't know if I can use it in my games or not, but it's surely worth my looking at it now and again.  Prices ranging up into the hundred thousand gold area are a great way of telling people "No you may not" while letting them think they still could.

Thank you Mog.

Lyxen is right about that.    Small towns should not have +3 weapons at all, let alone for sale.

I'm not actually that much a fan of Sane Magic Item Prices, but in general if you could simply use two items and it would work just as well as the combined item (so not more than one of them requires attunement, and they don't apply to the same checks), it's balanced to just add prices together, but if they're cumulative you're probably looking at a step increase in rarity, or possibly more than one.

Perhaps a slightly more direct answer is that a Weapon of Warning +1 does not exist unless you create one. Stacking enchantments is not a thing in 5e. Some enchantments such as the Dagger of Venom have a bonus to hit and damage baked in, but others such as Flame Tongue are meant to be viable alternatives to something like a flat +2 weapon.

Unless you are ready to go into homebrew territory, your players need to choose between the enchantments that exist.They cannot stack them.

I personally feel that stacking enchantments onto a weapon is fine but it will increase the cost exponentially. It would be quite complicated and time consuming to fit multiple magical effects on an item. For example (these aren't actual prices) a +1 weapon may be 250 gold but with another effect it may be 400 and then 600 with another.

If I were to do this, I'd scale much more harshly than that. Combining two enchantments on a single item is a huge boon because your attunement/equipment slots are the main limiters of your power in a campaign with easy access to magic items. If a +1 was 250, a +1 with any enchantment that requires attunement should probably be more like 1000 at least, and combining two attunement enchantments - well I probably just wouldn't allow them to combine at all.

Remember the game needs to be fun for the DM too. When you start letting players run wild with power it can be really difficult to continue to build encounters that challenge them, and as you scale the power up more and more it gets easier and easier to overstep just a little bit and up with a TPK.

I was only suggesting these prices as an example, I wouldn't want to give an exact answer without know what kind of game they are playing. I personally get enjoyment from the story and my players discovering more of it, with them solving puzzles and coming up with plans to get further and I find my players like being powerful, so if they are a little bit over powered it doesn't matter. Again it depends on the game, the players and the DM. Though I am a fairly new DM so I don't claim my ideas a perfect and your perspective was definitely an interesting one.

Sure thing. Didn't mean it as criticism, just my take after experiencing a high-level, item-heavy game. I generally give out homebrew items that are more powerful than official material, but I still lean heavily on that attunement limit to keep things from spiraling out of control.

You made a good point and it made me realise what I had said wasn't very clear so I just thought I would reply and elaborate on my thought processes. I think you are right about the attuenment limit. It has forced me down a specific playstyle route for my campaign but none of us mine so it is fine.

Quote from Mog_Dracov >>

I disagree.

1) The idea that there won't be a magic item store is foolish.   It breaks the realism for no reason, and does not make the game more fun.   There are lots of ways to restrict stuff without shouting NO, NOTHING.    Make it hard to get and more expensive at each level than what people typically use.   Put in taxes, guilds, etc.      

2) I love "Sane Magical Item Prices"  Link.   If you want magic items to be more expensive or less, I suggest applying a set factor (i.e. x2, x  1/2) etc.   But this is a well thought out relative pricing for everything from PHB, and maybe a bit more, do not remember.

3) If magic items cause problems, or just to screw with the players, I suggest you also adding some ways to lose the items.   Thieves exist.   Cities might confiscate a Wand of Fireballs. "You brought WHAT inside our wooden city?"    Use Disarm against them.   Throw in an "Greater Rust Monster" that works on magic items.    

4) Curse the crap out of them.  Especially with one of those "Hm, maybe worth it...." such as +2 weapon that sings (badly),  ruining surprise.  

 Sane magical items I use as a guide, but it is generally too expensive over all. For instance cloak of elvenkind at 5000 is far to high, but I do then use the list to help with relative pricing. So the cloak according to the DMG should be 500, I made it between 900-1100 in my world. I also limit the stock in magic shops, but anyone who states they ruin the game in my opinion is wrong, the one campaign I didn’t have magic stores the campaign was much poorer for it.  

The Cloak of Elvenkind is one of those nifty items in the Non-Combat section.  Those items are singled out because how useful they are depends very much on the style of play.  The Cloak of Elvenkind in particular gives a boost to a skill that causes a lot of arguments.  A player can use it to avoid a lot of combats, which may not be what the DM or the other party members have in mind.

As for the Magic Item Store, well, I guess it's all a question of how you want magic items to be perceived in your world.  I want them rare.  I want players to value their items.  I want that simple +1 Longsword to be an heirloom that might be passed down for generations, rather than something you dump after tier 1.  A simple store that you can wander into and just toss out gold to purchase takes all the fun and excitement out of having the item for me.  It also cheapens everyone else's items if the stuff they toiled to get is comparable to something the character with a contact in the nobility picked up on the cheap.

I'm leaning more towards letting players have consumable magic items now.  Possibly at prices less than the Sane Item list has them.  My main concern is, and has been, to keep my players from making them for themselves or selling them.  If I do as Xanthar suggests and make a mini-adventure out of getting the formulae and or components, I should be able to keep that under wraps.