The cost-per-use of an Identification Kit is 4 . Identifying a "white" item will increase the item's value by ~10%. Thus, it is usually beneficial to identify items with a base value greater than 40 if you are planning on selling them to a merchant.
Spending reward trophies on Identification Kits is not recommended, except for Monastery Credits and Battle Commendations (the first collectors in each campaign). The other collectors all offer Superior Identification Kits for 3 of the same trophy, which is a much better deal.
Trading 1 trophies for a standard kit (25 uses) gives a value of 25/1 = 25 uses/trophy.
Trading 3 trophies for a Superior kit (100 uses) gives a value of 100/3 = 33.3 uses/trophy.