I think that the reason why power kits from the shop are so expensive, is that those kits are the only ones that have enough power to be useful for players with max-fused items. While it is now cheaper to get power kits from boxes, SM makes money every time someone buys them. Also, many players don't have the credits required to buy up a lot of boxes, so they'll buy kits from the shop. The power kit shop charges premium price because fusion is something everyone needs on their mech, and buying it with tokens is convenient. Nice idea, though - would be better for non-paying players who only have a limited amount of tokens to spend.