Best I've seen was 4 years ago on a DFW-MEX flight that was oversold by 20. With very few volunteers, the offer went up to US$1,300 in a voucher. I was on that flight and I could not volunteer because it was the only time in my life that I flew on a Non-Rev ticket booked in "E" (if I remember correctly). Since I was on my way back from a visit to AA's Corporate Headquarters with a ticket paid by them, the CSR tried to get me off the flight and she couldn't. She told me she needed a high rank supervisor to approve it on the system...