Around here a new S&W 686 or 686+ goes for over $500. Used you can get them for $400-450. Depending upon prices in your area a used one should be in your range, or for a little extra (again depending upon how prices are around you) you may be able to get one new. I don't own one but the reputation is stellar.
I recently bought a used, but supposedly great condition (I don't yet have it- darned MD one a month law), S&W 586 for just about your limit including shipping and FFL transfer fees ($345 for the gun, $15 shipping, and $40 FFL transfer= $400, many areas you can easily pay less on the transfer). These are supposed to be great guns (the same as the 686 but blued instead of stainless).
Another Ruger GP100 would be around your budget. Here a local dealer has one blued 6" model for $404, the rest go for about $420-450. Again, this is a rather expensive area (usually) for guns so you can probably do a bit better. Since you say you already have a GP100 I don't know if you'd be willing to consider another (if yours is blue would stainless be different enough and vice versa, or would a different sized barrel be different enough...).
A Taurus 65, 66 or Tracker would be right on (or below) your budget. Taurus STEEL framed revolvers are great guns. Mechanically my little 605 is perfect, my 82 that I had was great.
For something a little different a Colt Trooper seems to go around that price range. They seem to have good reputations too. May be a bit hard to find though. So long as you stay with sellers with good feedback would you be willing to buy online?