My idea would be to bid as you do to say 15 tokens limit and then say how many further tokens you would pay for a player (this would need to be programmed into the game)each player not knowing what the other would bid.
Who ever bids the most wins so you can not lose more than the 15 tokens should you lose.
The game benifits from the people who can offered the extra tokens while the most a losing bidder loses would be the 15 tokens.
Players hopefully could still win by placing the odd token.