You make a 1.5 percent from the price you bought it at, not the price it was bought from you. Price goes up 5 percent every hire, rounded down.
So maybe profit=[ROUNDDOWN(current price/1.05,0 )]*0.015? I'll have to check that against my logs and see if it holds up.
To the person asking what figures in first, I think it's your current troop strength, then percentage based bonuses, then static bonus from equipment and allies, then pots last. I may be wrong on that though. Check some of the sticky threads in the strategy section, it's in one of them.