Well, considering a NIB rail gun is ~1000, it is certainly a fair trade, considering it is a used .40sw Sig with just 300 on top. Regardless of how much someone likes the round, it is really hard to argue that .40sw used firearms generally demand less $ than 9mm, and used Sigs in generally are usually had around the 5-600 range, even if in 9mm.
The RG is a great gun, it has a match barrel, and one of my favorite features on 1911's these days, and undercut trigger guard. Makes a huge difference for something so seemingly small. Like one of the previous posters stated, there is essentially two versions, with the "newer" version having essentially a beefier rail.
However, even on their website (
http://www.colt.com/Catalog/Pistols/RailGun%C2%AE.aspx) it only shows the stainless version has having the beefier rail. IDK if that means that they figured only the stainless version needed the beefed up rail, as the Carbon Steel version was strong enough as is, or if they just didn't change the picture for the carbon steel version to match the 2015 production.