One reason Colt nickeled Pythons often are less valued, is a persistant rumor that nickeled Pythons were guns that the polisher had slipped on, and botched the finish. These guns were nickeled since that "would cover up the botch".
This is complete BS, but rumors die hard.
The truth is, that for whatever reason, people just weren't fond of nickeled Pythons, or any other nickel plated gun for that matter.