How do we value our cars?

If its an agreed value policy for $20k, then fight tooth and nail to get that. If there is no agreed value, make them give you comparable cars in a list of at LEAST 5 cars, to your car, that they base the offer off of. If you don't have an agreed amount, I don't see you getting much more than 5 grand, hate to say.
I always say if it isn't hurting you to let the totaled car sit in a storage lot, let it drag out as long as possible.
BUT, I've seen crazier things with car insurance companies.