Well in your price range for an o/u, you'd be looking at a used "B" gun, either an older Browning Citori with 28" or 30" barrels. You might see a Beretta 68x with Mobilchoke barrels in this range if you're lucky. Current Trap and Sporting Clays trends call for a longer barrel (32's and even longer) so you can occasionally scoop up a shorter barrel gun for a better price.
I also think that the Savage Milano is a nice well made gun as well (Made by a F.A.I.R. - one of the Rizzini brother's) but they aren't in production anymore. They'll occasionally come up on the used market. They'll generally have a 28" barrel however.
For a new Over/Under - and one that's worth buying in that price range, a Huglu or Franchi, would be my choice. CZ (made by Huglu) Has the
Redhead Deluxe or even the
Redhead Target would probably be in your price range. I know Wolverine Supply carries the Redhead Deluxe, and could probably get the Target. Lebaron carries (or at least used to) Franchi, and the
Instinct S/SL would fall in your price range. That being said the Franchi is a lighter gun and would probably kick a bit more than the other options.
In a Semi Auto I'd take a look at finding a used Beretta
A400 Xcel (if you like blue) or a
AL391 of some variety.