Or, for fun, you could handle it differently: require the feat Craft Magic Arms and Armor.
As a DM, I'd be willing to consider a monk's body to be a masterwork weapon. The process of crafting the monk's fist (or foot, or knee, or whatever) into a magic weapon would be grueling, involving tattoos, burning, treatments in rarified acids, and the like. It'd hurt a lot, and do a lot of (healable) damage.
In the end, though, the fist (knee, etc.) would be a magic item, incidentally attached to the monk. Note that, for extra fun, I'd rule that it'd still work as a magic item even if detached from its original owner....
Note that this strays pretty far into uncharted rules territory, but it has two advantages over the permanancy option (which is also not covered in the official rules):
1) It puts the cost of a permanent magic fist much more in line with the cost of a permanent magic weapon; and
2) It's way cooler.
Daniel