So I was in a guitar shop and they had a group of 6 Gibsons.
Each one had a tag saying "Campfire Guitar." There were Les Pauls and ES 335 ranging from $2600 and up. Each of them was marked down to 800 to 1000 with the comment that they had twisted necks and their in store warrantees were void.
How expensive is it to fix a twisted neck?
Varies a lot from instrument to instrument. Somebody would have to likely pull the fretboard and hope there's enough leeway to shave the face down "flat", deepen the truss rod route accordingly and hope the neck stabilized enough that it won't continue to twist after you did all that work.
For somebody looking to get a deal, all the work involved would likely put you in the approximate range of the non-'campfire guitar' price-tag, maybe save a few hundred bucks here or there, but you'll have no warranty and more problems can arise. Toe an extremely competent repair guy or hobbiest, it'd definitely be a neat (albeit expensive) prospect.