Well, technically you pay the same price per man, yes. Due to the huge unit size the upkeep doubles in comparison to standard size though. Since your income is not affected by your choice of unit size, you may be able to pay for the same number of men but you are able to pay for only half the number of units. So, while fielding technically the same number of men (roughly), the number of units sustainable halves, resulting in smaller stacks and more 'small scale' engagements.

Anyway, since time is an issue too, retraining becomes a valuable strategic asset. Come to think of it, it becomes even more of an exploit against the AI than it normally is already, since the AI does not retrain as far as I know.