The important thing is who did the work and if it is work that you wanted done. I can't think of any well known 1911 smith who would work on a Taurus.
If the work is done by an unknown or the prior owner, I'd reduce my offer to the value of the used gun minus whatever it would cost me to reverse the work...or have it redone.
I have a 1911, built on a Colt 1991, that was built by a up and coming pistolsmith, that would likely have sold new for 4 figures. I didn't pay that for it and I don't plan on selling it, so it really doesn't matter what it would sell for...that is the way you have to approach having custom work done