When I checked in on Delta tonight, they asked me to bid on what I'd want to be bumped. They try to benchmark it by letting you choose between 25 and 125, or input an amount. So I input 300, bec I think that's what they gave me last time they bumped me.
But really, it depends what the other option is. Is the other option another flight getting in at the same rough time or a flight the next day.