If you have all the time in the world, tell them their offer is unacceptable, and to give you a call when they grow a brain. Eventually some supervisor somewhere is going to get unhappy about the old file, and maybe they will be more reasonable. The other possibility is to find every Volvo ad in the universe and show him what the cars really sell for, and how mileage does not affect a volvo the same way it affects a Chevette.

The third possibility would be to avoid beating your head on a brick wall, take their check, and tell them to go f*** themselves. The third option might be the least painful in the long run.