A magus probably would only hire someone's help agreeing to pay with one season acting as a trainer or teacher (meaning, for him, wasting one season of his own time) if the mercenary's service saves him at least (and probably more) than one season's effort.
So just check the recent thread about Services, trade and business between magi. A mercenary magus would probably be someone who can do, with ease (or effort, if he really wants that training or teaching to happen) something of that list that his employer wants and that would cost him 2 or more seasons to achieve.