Tigers should be rare like they were on the battlefield. Shermans shouldn't. The Firefly was a relatively uncommon model, but we don't have a regular Sherman.
More important, the real purpose of perks as I understand it is to keep historically rare uber planes and GVs from dominating the battlefield. A 262 isn't worth ten F4U-4s, but if half the arena flew 262s the game would be radically changed in a way it wouldn't with more 4-hogs, or with 262s existing but being rare like they are now, and anyone flying non-perked prop fighters might as well stay in the tower. Same for the Tiger. If the perk cost of the Sherman went way up, you'd start seeing a lot more Tigers, plus the Tiger would be all but unbeatable *except* by other high-perk tanks. Panzers and T-34/76s would become useless, just slow M-8s. The Sherman doesn't have anything like that effect because it can be beaten by Panzers and T-34s with favorable circumstances or a little luck.
A better fix IMHO would be to keep the current perk costs but make the armor on the Sherman less effective, more or less equivalent to the Panzer IV.