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Well, you have a desirable trailer in extremely poor condition (I personally do not consider a trailer that has been gutted more attractive than one that is still in one piece, but as-found). Please post some pictures of the exterior so we can better appraise.
That being said, I just read a thread where a guy was willing to pay $10k for a really beaten 70's era Argosy, so I guess prices are crazy inflated, even for trailers in really poor condition.
Anyway, another resource is to look at eBay completed listings and see if you can find some trailers similar to yours, and see what they actually sold for.