enchant NPC is a good idea,just make it hard to enchant it after +10-12!for example to make +3 item it should be free,for +4 10items(just an example) for +5 15items for +6 20items...etc...but after +10-12 u will need much more items...if +10 needs 50items for +11 u will need 100,for +12 150 for +13 300 for +14 400 for +15 600 for +16 1000 :D or something like this!