The american at gun is prtty good, when it hit's vet 2 it get`s higher range and if i'm not mistaken additional sight. With the at rounds the 57mm is able to penetrate even the heaviest vehicles. I don't really see the problem here. And making the bazookas cheaper is no option, cause you are able to equip every unit on the field with it.
The fact that bazookas cost the same as shrecks says it all.
Also, show me a replay where an AT gun reaches vet 2 and stays alive to be useful.