I was planning on getting a gooseneck hitch that pops on the rails I have now. I like the idea of having the options of pulling both a 5th wheel and gooseneck when needed. I'll probably need to borrow a different neighbor's livestock trailer at some point soon, so having the gooseneck hitch is a good idea anyway. There are a couple on ebay for around $140 shipped. Seems like a pretty darn good deal to me.
I like to work backwards as well. If I don't have comparable used prices, I'll go 1/2 of new and go down from there. In this case there are some decent options to compare to. I'd probably figure fixed up to sell around $2500, maybe $3000. The economy stinks and the gooseneck limits the buyers. I feel very comfortable paying $1000 for it as-is right now. I just figured I'd throw out $750 to see how you guys reacted...and I do think 1000 might be a better starting point if for no other reason than psychology of numbers.
The guy also has a two axle deckover bumper pull that is homemade. My friend said he might be interested in that. So I may be able to work out something on both trailers. Then my friend could just sell the 40'er for whatever he wanted.