you hire merc for hire price and you pay that price 1 time. other(smaller) price is mercs wage for 1 battle and you pay it only if you take merc into battle.
Mrcs can help you fight, but they take half of xp and wage for battle. so you must decide on your own if its worth. Maybe if you cant pass some quest which gives nice reward. or to fight at harder locations where half xp is more then you could get alone in easier locations.
