Champions should be charismatic heroes joining your cause for a reason they believes in (salary could be one of them) - not mercenaries you hire paying them once and then endure whatever you make them do for the rest of their immortal lives... or are they just slaves you buy from the owners of the locations you find them in?

Give the champions a loyalty stat and some personality traits that will make them leave you if your actions/special event/conditions don't please them. For example you get other champions they can't stand around for some reason, not completing their quests in time (the one you got him with and other ones they throw at you at level up or on special events/conditions of your sovereign/faction/enemies)... read more on the Champions as Quest Rewards thread (or moderators could move those posts here as they really more than just about the hiring quest).