I would say to him that there is no way you could probably come close to offering him what it is worth to him with all the miles and memories it must hold for him. Then offer him $2500 to start as that is what a decent running vehicle might cost. If he takes it fine, if not ask him what he thinks it might be worth. If he thinks a lot of you he might come back with a lower price.
The figure you have to decide going in is what is it worth to you.
