Note, however, that you probably won't be able to get the discount rate for such a flight, since all the seats in the discount coach fare class have probably been sold. Also, if a flight has reached the overbook limit, you won't be able to buy any ticket for the flight, except perhaps a full fare ticket. If the airline still has plenty of coach seats a day or so before the flight, it is unlikely that they will bump. You can also ask for a food voucher. Dollar-denominated vouchers are not subject to tax, so they stretch further.
Amounts depend on the degree of overbooking of the flight.
United sometimes will also issue a dollar-denominated voucher. United bumps more than average, Delta less. If you are bumped and the next flight out is the next day, the airline may offer you overnight accomodation in addition, especially if you are bumped while away from home. The bags will continue on to your destination and wait there until you arrive. So if you're planning to be bumped, bring enough clean clothes in your carry-on to last you a day or two just in case your bags are lost or stolen by the time you arrive, or you get stuck at a connection.