Most likely the gun you saw was a factory nickel plated gun. There is a theory that these were cosmetically the worst guns from the line, and were plated because it was easier to hide flaws than witha blued gun. I have stripped enough of these to know that they usually have more tool marks, but that might be because they were intended for plating, and more prep was not necessary.
The nickel Series 70 guns generally are worth less, that is when we're talking 5". Commanders, though not marked Series 70, will actually bring more to "collectors" if they are nickel. Anyway, though I see nickel Colt MKIV Series 70 guns listed all the time for $800 or $900, they usually sell for the $600 - $700 range. $549 is a good price.
They came from the factory in two finishes. I think older ones have wood grips, and the whole gun is nickeled. Newer ones had Pachmayr grips, a more "white" looking nickel, and certain parts were blued, like the thumb safety, grip safety, trigger, and slide stop. Don't take my word on the two versions and when they were made, though. Here are some pics of the types commonly seen.