Bombers don’t average 3.5 in damage, but rather 5/6 times that amount since they only do damage if they survive the AA. So the average SBR mission against a guarded factory nets [5/6*(3.5) - 2] IPCs. That is less than one per bomber and hardly seems excessive. You are probably better off using them for other purposes than SBR. If that is the case, it is hard to see how the Axis or Allies are significantly disadvantaged by them. Campaigns heavy on SBR do seem to turn the game into a bit of a crap-shoot though. 6 bad rolls either way and the side rolling poorly is hurting.

I assume bombers were made cheaper as a defense to cheaper navy. They seem fine to me although I never play with the Heavy Bomber tech. It just seems to make the game into another kind of crap-shoot. Without tech, bombers are strong but every country can afford them.