Well, on CL you'll see everything from take it down the wood is yours to $20k or so for large standing barn frames in mid atlantic region. Several companies in OH, PA, VT etc that do this already. In those markets you have hundreds of thousands of old frames, less termites, great frames.
I've looked into this myself and I'd be leery of paying more than $4k for almost anything just because you can get a good frame for that if you wait. Now, that said...there are a few special ones out there. Really old stuff, pure chestnut, that type of thing, saw one that was solid walnut in southern Ohio. Then you might pay a bit more.
I'm a bit dismayed at how they just chainsaw up the joints though. Lots of good reading on the web on pulling pegs, drilling them out etc.
Just don't overpay and look hard. The other problem is that in LA and other parts of the south is that the war between the states sort of gutted a lot of barn structures.
Good luck, it would be fun!