TTG is probably right, maybe even a little more if it's in excellent condition. You don't see to many M455s come up for sale any more. I have a 6.5 inch model and really enjoy shooting it. They came with 5 moon clips, I don't know what the availability of those moon clips is right now. The gun should function fine without the clips but I like using them in mine.
I paid about $500 for a 4-inch 455 (NIB) about a year ago. I think it had been in the LGS owner's private collection NIB for years, and still had the brown packing grease all over it. I'd do it again if I could find another NIB. It's a great shooter. They weren't produced for very long, and were discontinued some years ago. They come up used now and then on Gunbrooker, etc. in the $400 to $450 range for good used ones.