Craft Magic Arms And Armor [Item Creation]
...you must use up raw materials costing one-half of this total price.
Longsword +2 requires material worth 4000 gp (+mw cost).
If your player asks you what these materials are, what do you say?
I was thinking about going through the easy way:
You need certain kind of gems in order to create magicals items. No gem is ever worth more than 1000 gp. Therefore the crafting process will consume maximum of one gem per day.
Different magical items need different sorts of gems, so no one can stop people from making magical items simply by destroying certain types of gems from the world (through a Wish or some divine intervention).