you do get a ratio of gold based on the power of the opponents *killed*. Wounding doesn't count.
This is a "display bug" during the fight. Your actual bank balance should increase only for the 5 heroes.
Same payout for all participants based on the point value of what's completed. If you don't finish the whole thing, this is the only time where it matters that gold skulls are 5pts vs silvers at 1pt. Those points are what determines daily payout.