I have a player who wants to make a fake treasure map that he can sell off after spreading rumors for the required amount of time. How do other DMs handle skill checks, costs, etc... for making a good counterfeit item? I was thinking of making him make several slight of hand checks the average of which will be the DC for NPCs to beat to realize it’s a counterfeit, then I was trying to figure out a good cost for him to creat the map? I would appreciate any advice.

Thank you

Well I think the proper tool proficiency would be a forgery kit or cartographer's tools. As for cost I would ask if they want the map to look Common in quality or Uncommon and price it accordingly. 

Seconding Ophidimancer; the appropriate proficiency for this check would be the Forgery Kit. If the character isn't proficient in that tool and/or doesn't have the kit to hand, then it'd be a straight Int check to determine the DC for detecting the forgery; I'd also have them roll only once, instead of multiple times for an average.

As for the cost of creating the map, I would make it no more than the cost of some paper, ink, and a pen - so about 10.22 GP (i.e. 10 GP, 2 SP, and 2 CP). However, I'd also rule that it will take a good amount of time to complete the forgery, at least a couple days.

I thought the forgery kit just allowed you to creat a forgery, I don’t remember it specifically designating a skill to use. He wants to take an old piece of ship wood & create a map that appears to lead to a sunken treasure. 

for me it cwould be as simple as buying parchment paper. not something thats really hard to get, ink and pen or quill. again cost wise its almost nothing really.

the hard part would be from skill checks. i would allow him to use forgery kit which literally are made for that very purpose. and i would use intelligence as a stat.
i would then use his roll against any insight to those he wish to sell it to. it would be as easy as that to make.

Quote from Raffen >>

I thought the forgery kit just allowed you to creat a forgery, I don’t remember it specifically designating a skill to use. He wants to take an old piece of ship wood & create a map that appears to lead to a sunken treasure. 

 If they’re getting proficiency on the check from the tool proficiency, any skill you might put the task under is irrelevant. You only add your proficiency bonus once, regardless of how many sources you derive proficiency from.

Bottom line is that if you let the player use their forger’s tools, you don’t need to think about skills at all.

I would have them use either forgery or cartographer's tools, or even woodcarver's tools. They don't have to be proficient, that just gives them a bonus to the roll. 

But....I would certainly require more than that. To really sell it as legit, I would use some of the downtime revisited rules from Xanathar's. Things that could impact the believability of a fake map could be:

1. Research. Did the PC do some research to know ship names? Have the landmark names changed recently? Does the map reflect this?

2. Carousing. Normally carousing let's the PC pick up some lore, but could easily be modified to plant some lore. "Hey...did you hear? Some sailor found a map to Macguffin's gold. What? Never heard of it? Let me enlighten you."

3. Selling a magic item. I realize the map isn't a magic item, but it could lead to them. And the complications given in Xanathar's line up nicely to someone selling a treasure map. (If the intent is to sell it.)

Using these (or others) can also allow multiple PCs to join in and roll some dice as well. The more successful each is, the more believable it is that the map is real. 

I like using a 3 rolls system for activities where there is a risk involved like this. Each roll uses a different relevant check (something like INT (tool), WIS (survival), and CHA (deception) in this case). Set the DC depending on the difficulty and how much they stand to gain (maybe around 12-13 for a 100 or so GP). Then the number of successes determine the amount of success:

• 3 successes = major success, full reward, not caught
• 2 successes, 1 fail = minor success, half reward
• 1 success, 2 fails = minor fail, no reward
• 3 fails = major fail, no reward, guards alerted

The minor success/fail could have repercussions later, but not at the time. Severity of the repercussions depends on the severity of the task attempted (for example a few weeks in jail or a fine for minor offense, or may make an enemy/rival). The 3 roll system is good for things like burglary, gambling, and other risky/illegal activity.

To produce the map they should use Deception skill, Forgery kit and Cartographers tools. Roll with advantage if they are proficient with two or more of those. The results will set the DC to detect as a fake. These skills/ tools were selected based on, what makes a good lie, how to make a document look legit (and weathered) and what makes a good map.

To detect as a fake use Sense Motive and Cartographers tools.

You may also apply bonuses to the production and/ or sale based on how they go about making and selling. For example if the players find an old weathered document as a base and erase the old writing it will look more convincing. Asking questions like "is there a hill with two large pillars nearby, how about a temple next to a lake?" as if trying to find the location on the map themselves will attract attention. If people think they have a map days or weeks before they even mention selling it then the sale seems more legit.

r

I'd think through how exactly they're going to sell this thing as legit, and make checks accordingly.

Probably one check to make the map - I'd be fine with any of those tools or skills, though, because I don't think the quality of the map is really the hard part. After all, the treasure map isn't expected to be a super high quality map, necessarily, right? So that's when I'd let the player suggest to me whatever skill/tool combination they want to use here. They probably need a good skill/tool/roll to get a map that convincingly looks really old, but if they're not trying for that then it's easier.

But then, how would they go about convincing someone it's a legit treasure map? You can't just walk up to somebody and say "hey, I've got this map. It's a treasure map, I swear!" So they have to plan out how they're going to plant rumors, how they're going to create the idea that there IS a treasure map and that they have it. There's probably going to be many steps along the way, and for each of them you have them make an appropriate check. Probably going to be some Deception in there, maybe a Performance or Persuasion. But the player might come up with interesting ideas! Like, maybe first they want to find the town rumormonger, the best person to "accidentally" leak a rumor to. Insight check to find who that is!

Definitely make sure that it's not "one failure = whole task fail", because if you're having them make many rolls a single failure is very likely. So see what they fail at and adjust rewards accordingly.

Quote from SagaTympana >>

Quote from Raffen >>

I thought the forgery kit just allowed you to creat a forgery, I don’t remember it specifically designating a skill to use. He wants to take an old piece of ship wood & create a map that appears to lead to a sunken treasure. 

 If they’re getting proficiency on the check from the tool proficiency, any skill you might put the task under is irrelevant. You only add your proficiency bonus once, regardless of how many sources you derive proficiency from.

Bottom line is that if you let the player use their forger’s tools, you don’t need to think about skills at all.

 you still need to choose a stat to go along with it.
that is clearly written in the tools descriptions.
so basically, the kits are skills themselves. except they are not tied to a specific stat and are left to the DM to tell you. in this case i'd say its intelligence.
i'd make them do an intelligence check and if they have proficiency with forgery kit, yeah add your proficiency too.