It's an interesting idea but I think there's a good deal of variability in this.
In my experience, for instance, adventurers of any level are typically willing to join a like-minded party for a more or less equal share of any loot. Of course, they are free agents (ie they don't just follow orders) during the adventure but when you're only hiring one guy out of the mercenary's guild that's really what you're getting anyway. The follow orders types are in companies that don't just hire themselves out one at a time. (In short, I don't see why NPC adventurers would be treated differently than PC adventurers in this regard).
There's also the possibility that depending upon the mission, an NPC might join the party for free. When I was running my campaign in California, an NPC (Ranger/Paladin) heard that the PCs had uncovered an evil temple. His response: to find the PCs and find out where this temple was. He then headed down into the evil temple under the city. When the PCs said they were doing that, he said "good, some company will be nice." He certainly didn't charge the PCs money--he was going to be heading down there anyway. (And he ended up being the only character resurrected in that game--the PCs used the scroll of True Resurrection they found to bring him back from the dead after he died fighting a demon with them. My players were neat like that).
In the same way, I imagine that a rogue who wants to steal the crown jewels of the evil king might volunteer to help the PCs sneak into the castle--and use their activities as a diversion to cover his heist.
Given the variety of NPC motivations and the ability of high level NPCs to pursue those motivations as they will, I'm not sure that a flat fee for 15th level mercenaries is really a useful thing to have.
