IMO it all depends. Personally, I love the 55.1 series, for the look and the mere fact that it was the last of a great era of trucks. So to me, I have always valued the 55.1 the most. Maybe even more than the first 47's. There is a modified one in my area for 8500, I can see it being close to that with all their mods. But you have an OG truck here (title I am assuming). So to me, if it ran and I had the money, depending on condition I might go 4000. But that is me. I would want to modify it to some degree, to make it an everyday driver. Some would want to restore it and leave it to collect dust. So they might pay more. I have seen running trucks go for 2k and as much as 6-10k asking prices. It all just depends. It doesn't seem like these trucks have a steady value. So I would say you could go from 2000-4000 with out seeing it. But again, that is just me, depends on the seller and depends on how bad you want it.
