Well, a rational approach to this would be to make a deal with everyone present that one person will make the starting bid, no one else will bid at all, and the bid as well as the 20 dollar bill would be divided amongst the party.
With twenty people present, everyone would benefit 95 cents, so it would be just as sensible to simply let someone make the starting bid, and let him benefit 19 dollars.
Of course, in contests like this "winning" is not the same as "profit".
Or, in other words: If the people in the "auction" would consider them as one participant in the contest and the auction keeper(s) as the other party, they would see that the only way for them to win and the auction keeper to lose is if the total bid stays below or at maximum of ten dollars in the case of 20 dollar bill.
Instead, they fall into the pit hole of thinking each other as their "enemies" and try to win at that contest instead of seeing the obvious - the only one who will benefit when the bid goes past certain mark is the one who is auctioning the 20 dollar bill.
Turambar also falls into this sort of thinking. If you switch your paradigma from "me versus others" (me vs other bidders) to "us versus them" (bidders vs auction keepers), the contest gains a whole new meaning.
In ideal case, no one rises the bid after the opening bid. In no case is it beneficial for the bidders as a whole to raise the bid over half of the prize money (with these rules). At any time when bid goes past half the prize, the auction keepers are getting profit.
This, of course, is limited to purely monetary definition of profit. There are other ways one could benefit from winning a contest.
"Look at me! I have so much money I can bid this much for essentially worthless victory over these other guys around here!"
Yep, I can see how in high society that could be beneficial signal to send out - and the actual monetary losses can still be negligible to the people I'm thinking of.