Does the offer increase with time? I have witnessed something similar. On a flight from SEA-SFO, I was offered $100 to take a confirmed seat on the next flight, or $300 to take a confirmed seat on the last flight of the day (about 4 hours from departure time). I could have "waitlisted" for all earlier flights had I taken the $300 option. I took the $100 option, because I didn't want to sit in the airport all night. However, another person (non premier) took the $300 option, and ended up on the next flight (with me), with $200 more.

I have never seen any increase in dollar amounts offered besides this. I know that they do give more for International Flights (LHR-SFO is a big one) -- like $500-$600.