Did you talk to the GA when you got to the gate? If so what was said about $$? Over Thanksgiving my wife and I bid $800 for some flights and in one case we were told they got the volunteers they needed at the gate for less than $800 but only after I approached the GA. Nothing about the online bid is binding. I have seen GA ignore the electronic list entirely and start fresh at the gate, I have seen them follow it religiously, I have seen them use it as a suggestion. The GA have total discretion on who they pick and the agreed upon terms (compensation, routing, upgrades, etc.) within DL guidance. All that matters is what you agreed to with the GA in person. If the GA did agree on $800 you need to contact DL about the missing compensation, if the GA never verbally agreed to $800 you are likely out of luck. Most likely (guessing from experience here) the GA sought more volunteers at the gate and went to $500 to get enough. Normally everyone gets the highest compensation offered at the gate so you got lumped in with the $500 offer.
If you enter a bid online or at the kiosk you always need to talk to the GA as soon as you get to the gate to verify that the GA is actually considering you and to set terms prior to departure.