Where are you based? In the U.S., VDB usually comes with a free voucher or $ off future flights (usually domestic, must be operated by the airline, and depending on where you travel from/to, probably limited to the lower 48 states). Most here prefer the latter as it's money off a flight of their choice, inventory is less limited (bump vouchers usually come out of the award bucket which can be very limited), and of course can earn miles (free tickets don't). Rarely is cash offered (that's when they really want someone to volunteer).