I forgot to say, when you ask about volunteering early, include asking for what the compensation and alternate flight would be, prior to surrencdering your boarding pass. You can even negotiate at that time, but keep in mind that the more you try to negotiate, the more likely the gate agent will choose a different volunteer.
As far as involuntary bumping goes, the airline must pay cash if the customer demands that, although the dollar value of scrip (voucher or free trip ticket) is likely to be more and most likely only the scrip offer would include a hotel room.
I have never observed an instance where less than $100. was offered, usually it is at least $200. and usually there are enough volunteers for $200.