I paid $500 for mine, it was only a couple of months old and it was like new having fired only 50 rounds. It also came with 2 boxes of Ammo and all the paperwork etc. I thought it was a pretty good deal, given how much they were selling for new at the time including sales tax etc.
I've haven't seen many 4 inch ones for sale since I bought mine, but as they only came out in 2008 I guess they are still fairly rare on the used market. There is a 45 Colt for sale for $675 locally which seems excessive but Gunbroker has one for similar money. The vast majority of Redhawks I've seen have been longer barrel models, but plenty of people seem to be asking as much as $650.00 for stainless.
IMO $550 would be a decent deal for an as new one.
Although I do own S&W revolvers, with the great double action trigger the 4" Redhawk seems to come with from the factory and it's inherent strength, I wouldn't rush to pay a lot of money for a S&W Mountain Gun. I started looking at those before I bought my Redhawk and have never once regretted my decision to go with the Ruger.