For a voluntary bump on a domestic flight the normal NW compensation is a free-trip coupon. Depending on the markets you fly to/from, the free trip can be worth up to $500, but the flights are booked from "W" inventory and obviously don't yield miles.

The free-trip coupons can also be applied as cash vouchers worth $250.

For involuntary domestic bumps, the federally mandated compensation ranges up to $400 in cash, but generally the airlines will "negotiate" by offering you vouchers and hoping you don't object. If you do object, make sure you know the rules: Under some circumstances little or no compensation is actually mandated and the vouchers can be a gift.