From what I understand, Delta will only give out a MAX of $400 plus hotel and meals for a bump. If you are bumped with only a 2-3 hour delay, expect $200-250.
On a last-flight-of-the-night ATL-BOS a few months ago, gate agents needed about 15 people to volunteer. They had a list of potential candidates (including us), but when they wouldn't budge on the $400 compensation voucher about half of us changed our minds. This caused a lot of last minute confusion at the gate as they searched for others who would do it. It also delayed the flight for at least 20 minutes. So, I think this $400 max rule is pretty firm.