I don't know, I'm skeptical. Yes, you are right in every way about Mot. Infantry being far better than regular, but you should consider the cons.
An infantry regiment takes 500 food, 750 goods and 1,500 manpower right? So far so good. But once you meddle with the base manpower, it starts to get a bit... wonky. An Infantry regiment is 1.5k manpower. An infantry regiment in-game is exactly 1,500 men, as far as I know. Makes sense to account 1 man for 1 manpower. So should you lower the costs, the infantry should in theory become much, much weaker. 33% weaker, a massive blow. At this point a L.6 TBomber cam obliterate 1 regiment in one run. A Nuclear Battleship, albeit a unit used much less, also kills it in one bombardment mission. We're talking same HP as AT or AA. At this point I'd rather build those at lower costs to hold off a unit till air superiority comes in. I'm not going to talk about damage, because you can imagine what that side will bring too.
I don't necessarily disagree with your opinion on what should be changed, but the way I see it they (devs) probably want some sort of consistency when it comes to cost/durability ratio. Also, don't forget that infantry is still the most versatile unit in-game, and backbone too, considering it may be the only unit reflecting it's real cost in the game. They have no weaknesses in any territory. They can handle themselves fine as escorts for weaker units while your better units are out in front, city invaders, etc. I'd consider no disadvantages a good trade for higher cost.
I only considered Infantry in this reply, because the other units can get a bit touchy. I would just say 1.5k manpower as the default cost, but the trade-off is the oil/metals cost,and disadvantages. I won't even touch on Mech. Infantry, because whereas the 2 different Infantry units rely on manpower alone to determine raw durability (or so I think), they have actual armour, which you know is far better at stopping deadly things than flesh is.
Also, I would say the lower manpower on planes is OK. A plane wing isn't as big as a regiment. It takes less men to run. Same for AT, AA, Artillery, etc.