Day 2 of 9tails with everyone there we beat it in 11minutes. Day3 of 9tails with everyone there was 7minutes. Day 4 of 9tails, top 3 was missing a member, 5minutes. It definitly does not scale up. So you wont get more everytime hes killed.
Not debating on if this is good or bad. Just the numbers of coins. Exp depends who you were. for a top 5-10 person in 9tails you will be getting less (assuming he lived 30+minutes) for anyone else the bounties make up for it by alot.