Once upon a time I thought balancing via cost would work. Turns out players would prefer to pay the extra and moan about it, rather than use the weapons that were -10% or whatever.
Weapons cost were changed only to compress the difference in price for the highest tier and the lower ones, to reduce the amount of time it took to get the highest tier. They've not really been changed to reflect the power of the weapon as these haven't been settled yet.