Two DD's in the J! round have been for the [J! round] max of $1,000 when the contestants had less than that ($800 both times). http://www.j-archive.com/ddstats.php?season=goattournament at the bottom shows them all.
ETA: And for one DD Brad found in a DJ! round when he only had $1,000, he bet the max of $2,000. The rest of the DD bets have been for 100% of what the bettor had.