I don't think USD$400 is the minimum in the US.

Last year I voluntarily bumped myself off an AA MIA-YYZ flight. Prior to boarding I told the gate agent that if they needed more volunteers, I was willing to be bumped for the USD$300 voucher only if they could confirm my upgrade on the later flight. It turned out they needed me at the last minute, and so I walked off the plane in exchange for USD$300 voucher, free upgrade on the later flight (didn't deduct my e-credits), and a free visit to the Admirals Club (wasn't a member at the time). So I thought it was a pretty sweet deal - I used that voucher towards a ticket to SXM

Since then I haven't encountered any requests for volunteers on my flights. I'd say $300-400 USD vouchers are about standard for bumping. Some airlines will even give you a voucher good for a free coach ticket in North America.