Hello I'm running a 5e campaign, atm I have 4 level 5 players and they recently killed a giant clam (cloyster from 5e pokemon) and they managed to salvage half of its shell and have spent a lot of effort maneuvering it out of the dungeon in hopes of making a shield possibly.  I'm all for this and plan to give the shield cold resistance since the creature it came from had cold resist but I'm not sure what ballpark to put the crafting price at once they get to a big enough city with a capable blacksmith, etc. 

Ring of Cold Resistance is an similar Item, it is Rare, the maximum usual cost for a Rare Item is 5,000gp. With the Provided Materials I would knock the price down by 25%, so I would suggest 3,750gp. Ring of Cold Resistance is an Item that requires Attunement because Resistance is powerful, you should also make this Shield that gives Resistance Require Attunement or change it's effect to be more situational.

I would find the nearest similar item and use that as a basis for the shield. If the cold resistance is a function of magic then Xanathar's has rules for crafting magic items, how they require exotic components, and the rarity/cost/time commitment needed in the crafting.

The nearest similar item is Armor of Resistance, which is rare and requires attunement. (This will keep the players from just tossing the shield to each other in combat to pass the resistance around as needed). As they are 5th level I will assume the creature was of a CR that would be a mildly dangerous encounter for such a group (somewhere around a CR 5/6). Problem is that XGE recommends a 9-12 CR challenge for an Exotic Component for Rare magic items. So to bring the quality of the shield down to Uncommon (CR 4-8) it would be a good idea to include a flaw of some sort. Normally tacking attunement on an item is more than enough to do this, but our nearest item already has attunement (and for reasons listed probably not a bad idea to give it to the shield as well), so we should include some other flaw that limits it. Charges are useful for this letting it function a number of times per day then needing to recharge, other options include things like altering an attuned character's personality in some way (Adding a Bond, Flaw, or Ideal), or a situation where it stops functioning (cold resistance so in excessively cold/hot environments it doesn't function), or incurring a vulnerability (as it came from a clam/aquatic creature perhaps a vulnerability to lightning/electric damage). With its rarity brought down to the Uncommon range, XGE lists the cost for Uncommon Items at 200 GP to craft and taking 2 work weeks. Add to that the cost of the smith/enchanter (2 gp per day). And you have a cost of  200 GP + 20 GP (10 workdays/2 work weeks of skilled hireling)+ 10-30% markup (if the smith/enchanter charges more than at cost prices).

 If the cold is simply a function of the creature parts then use the basic crafting rules from Xanathar's. In which case it would be 10gp in materials beyond just the shell, take 20% of a work week, and cost whatever the smith charges for their time/effort. PHB says skilled hirelings charge 2 GP per day and it requires proficiency in smiths tools to make armor. There may also be a markup for the fact that it is a custom order and using non-standard materials in the construction. Which would come out to 10 GP (for the shield) +4 GP (for the smiths time) +10-30% markup (if the smith charges more than cost).