Take a look on the EE, Parts and Bits section, and see what you can find with the search.
I've seen prices asked for take off barrels ranging from $15 on up to pretty silly amounts, with somewhere between say $75 and $150 being a normal "asking price" range at gun shows. Whether anyone ever sold one at that price, on the other hand...
Tough one to call, as the barrel is pretty much an unknown quantity in the equation. Someone may be actively looking for one to return a botched "custom" to stocker condition, or they may be looking for a barrel to use as a hack to try out an idea with. Never know.
A search through the posts in the EE is probably a good bet to start, though.
Cheers
Trev