For Vol they can offer anything they want.
For Invol see DOT rules and it is cash/check NOT a voucher
Most people are too anxious to volunteer
If they held out they will get more, because the airlines do not want to do Invol and pay cash.
Also there is a DOT rule, that the Airlines DOT compliance rules have to be available in writing at the gate and they can be fined if it is NOT available.

http://airconsumer.dot.gov/publications/flyrights.htm