Topper's the closest to my approach; there are two ways to look at this sort of situation - the first is that he's the leader and your girls are indeed hired musicians. If that's the case, I would pick a figure (perhaps as low as $50, or as high as they can push it) per job. As long as they get their money, then it doesn't matter what the leader makes. (As I type this, I'm on a break on tonight's gig - while the leader on this corporate gig made thousands of dollars (I have no idea how much that is,nor do I care), I was offered a fee that made it worth my while.
if the guys doesn't want to do it that way, I'd go back to the classic union approach - the leader gets double what the sidemen make (with a trio, that would give the leader 50% and the sidemen would get 50% of gross; with a quartet or larger band, the leader would get a smaller percentage of gross, but would still make twice what the others make). In neither case does the cost of the PA come into the picture - that's part of what being the leader (at that level) entails - supplying the PA and (if out of town) transportation as needed. (Last night's gig was that way)
Or your daughters could tell this guy to take a hike and start a group of their own; I betcha a nickel that your girls are more than half of the reason that the band gets hired.