The extra cost of using marines, mountain or cavalry are trivial by comparison to the excellent benefits to attack and movement the division gains from their special abilities crossing rivers or hills and mountains. That production needs a lot of resources to run at full efficiency.

I find it hard to justify using vanilla infantry when spending so much on a very expensive battalion, heavy tanks cost 1000 production per battalion and heavy TD are 500. If you add the heavy armour to a foot slogger division the advantage is that you can add line artillery to make a heavy assault division, most of the soft attack will come from the artillery and they are made much more effective if the enemy can't pierce. I don't see the point of putting more than 1 heavy tank or heavy TD in any division, almost all the armour benefit comes from the best armoured unit. Especially with the 1936 start and if you have access to reasonable amounts of chromium. Heavy tanks are interesting and worth building for some nations.