I am going to assume it’s not factory. In that case I would appraise it at nominal $500. The reason I am fairly certain it’s not factory is because I have a fully gold-plated Detective engraved by Ken Hurst that he made in 1982. When I called Ken about five years ago after purchasing that gun, he told me that Colt at that time did not allow gold plating, and he had to do it outside of Colt. It was a special gun for a close friend, but he said it was the only gold Detective he ever did.
Check the gun for any familiar signatures, but just a random aftermarket gold plating, in my opinion, does not add value, and possibly even lowers it.