Unaltered they sell for around $1300. Changed to 455 Webley they sell for as high as $1500, no more unless someone put a fancy finish or something on it, but it's a hard sell. I don't personally trust guns altered to 45acp because you never know until you examine it if someone shot full power loads out of it.
With your gun, if the bore is good and the timing hand is strong with no play in the cylinder when cocked or with the trigger pulled and held down, I'd suggest $1300-1400.