How do I determine the purchase price for something I made and donated?

If you are trying to determine the value of a donation, your "purchase price" would be the cost of materials used to make the item. You can't include the value of your labor.

In the same case, the fair market value (FMV) of the item would be the amount you could expect to get if you sold the item to an unrelated person.

If this isn't quite what you are asking, reply with more details of what you are working on. 

In the case of something you made, you are not allowed to deduct the FMV, if it is more than your cost.  You are limited to the cost of materials you purchased.  FMV applies when you donate a used item. For example, you donate an old chair.  You can not deduct the cost you paid for it, only what it's worth today, as a used item.