Put in place a per player cap for how many coins one can earn per banish and don't worry about any cap on the boss.
This Yes! If 100 show up to fight it they all have equal chance at high payout. If only 10 show up same chance. Then if we end up with 6.25 Baal because we have a server full of people whacking at him we all get a fair amount for the work we do.