this is an issue with the way that NWN determines level requirements, the pricing lens takes the price of the stack of arrows, and calculates what the level requirement is based on that price
the game engine determines the level requirement based on the price of a standard nwn sized stack (99 arrows) and uses that price to determine the actual level requirement
thus to accurately price determine the level req for ammo, drop it into a stack of 99 and then use the lens on them
for the record UMD does not allow you to forgo level req's, just class, race, and alignment reqs