On the Tuesday after last Thanksgiving, flights to San Francisco were woefully overbooked because of bad weather there the day before. I volunteered out of EWR ($200 and upgrade), and was put on a one-stop through Chicago. Got off the plane "to stretch my legs" (and to see if they needed volunteers)...$200 more...third time the delay was longer (so $300), and no first class (remember, I had been upgraded!) so a coupon for a future upgrade. Net to me: $700, one upgraded segment and one future upgrade.

For what it's worth, United bases their (first) offer on how long until the next flight; they may offer as little as $100. American seems always to start at $300 regardless of how long a delay you're buying.

One I've always wondered about was at ORD, where the flight to Indianapolis at the next gate was oversold so UA was offering hotel and a free flight (back in those days!). It was only about 5:00, and I wondered if it wouldn't be cheaper (and faster) to rent cars for the volunteers and let them drive!!