Maybe instead of 'buying promotions', you could call it 'retraining', or something like that. 'Buying promotions', to me, sounds stupid, like 'buying wisdom' or 'buying experience'.
purchasing promotions, I think, is more realistic, however, it would fundamentally change how war is done in Civ. Instead of war 'experience' in promotions, the victory will go to whomever has the most $. Perhaps balancing this with +15% modifier if the unit is defending on his own cultural area no matter what?
Make it cost a lot, and implement my idea of units having a risk of conversion on someone else's ground, so that the more money you spend on promoting units, the less you are spending on cultural upkeep, so the more likely they are to flip once they are off your territory.