Collector value. Those are quite uncommon, if not 'rare'.
I'd suggest watching for an extra .357 mag. cylinder on E-bay or Gunbroker.com. Should be a swap in.
The gun parts are showing up from confiscated firearms that have the frames chopped (gun destroyed) and then the remains are sold as junk to folks who dismantle the guns and re-sell the parts.
Then, of course, keep the original cylinder handy for when you tire of the piece and want to sell it.
Just my $0.02 worth,
Terry T
edited to: Found one for you:
http://www.ebay.com/itm/RUGER-GP100-357-Magnum-STAINLESS-CYLINDER-and-CRANE-/261705063653?pt=LH_DefaultDomain_0&hash=item3ceed65ce5
Might not cost much more than the bore job.
Terry T