I was on an overbooked IAH-BNA flight a few weeks ago. My ears perked up when the GA started the announcement.... the offer?

A voucher for all of $100, and a seat 4 hours later. A few college age kids volunteered. I wanted to slap them.

Generally-speaking, $300 domestic ($250 if the delay is really short) and $500 international are when I start to listen. Of course the real heartbreakers are when they are really desperate; but you can't miss THAT flight. I still wince at the (NW) $1000 voucher I had to decline a few years ago. Ouch.