I know its expensive, hell.. I don't even touch it much. But when you look at it and compare to say, a Sherman. It might be worth it..
50mm gun on Sherman is 100 munition PER sherman
On Stug, if you get Veterancy 2 you get a Gunner for your Tanks.
300/50 (not sure of exact costs, to lazy to look up)
For permantly having upgraded stugs everytime they come off the factory floor, no upgrade times.