If you're even half serious about sport shooting (Trap, Skeet, or Sporting Clays), you should be looking at $1200 - $1400 used or $1500 - $1800 new as a starting point. The cheap over/unders will not hold up to that volume of shooting.
It hasn't been around long enough to have the final verdict in yet, but the SKB 90TSS looks like a good deal. It is available with either right-hand or left-hand wood and 28, 30, or 32 inch barrels. Its predecessor, the 85TSS was a very popular sporting O/U, and had a reputation as a durable shotgun, and was considered to be the most bang for the buck among entry-level sporting guns.
http://www.skbshotguns.com/Shotguns/90TSS.php