Don`t cast this in stone,but I`d offer around $5000 +/-.You`ll need more opinions for sure.Is the guy just leaving it to you to set the price?I`d tell him,"I really don`t know what to offer and don`t want to low ball him,how does $3,500 sound?"Ofering that way,I don`t see how he`d be ofended if he wanted a good bit more.Why start high?It then would be up to him to give you a value he had in mind.Don`t be surprised if it`s at least double what you offered.
If you do end up with it,first thing I`d do is read that badge on the fender so you know the proper name to call it.