I actually think mercenaries should only cost money if you dismiss them. While being a part of your party, they gain room and board with you, AND they share equipment and loot as long as they are members of your party. As soon as you dismiss, though, they should want a "share of the loot" especially if you take all their gear before dismissing them. I mean, isn't that usually how adventurers work? They'll join a party for a cut of the loot as opposed to some up front fee?
Possibly but I can foresee significant issues with implementing this.
When you dismiss them, can you just say "no" to paying the merc? Or does the mercenary just automatically steal X gold from the party stash upon dismissal? If so, the party could try to spend it all buying magic items before dismissing the merc, and thus having no money to actually pay the merc with.
Would the merc then steal a certain amount of loot from the party, gp + equipment + consumables? How would this be implemented: randomly? That could be incredibly aggravating.
And in both of the above situations, it becomes the optimal solution to just kill the merc instead of dismissing them, so you can reclaim your lost equipment and not have to pay their fee. It's all around just easier to have a gp cost upfront, that theoretically is essentially un-reclaimable from the recruiter - either by being a too-high DC to steal back, or (my recommendation) just disappearing entirely from the game.
