You greatly under estimate the price of tommies. IS aren't some crappy overpriced 4 man squad. They are extremely good even at 4 men. 30 is more appropriate, 28 is slightly too cheap but its not a big deal.
assuming that one tommies is equal to one grenadier, it still means that a 4 men tommies squad should be 240 mp, not 280.
the british pay extra in the beginning for the possibility of upgrading into a 5 men squad, in addition to the upgrade itself. The wehr doesn't get the benefit of a 5th man upgrade, but they don't have to worry about an overcharged squad either.