Like everyone says, you are looking at a Remington or Mossberg pump for about that kind of money if you want new. I got the Remington 870 but plenty love their Mossbergs.
What no one mentioned is that you can pick up a duo combination barrel Mossberg 500 in CA at Big 5 Sporting Goods for $279; which is a sale price they advertise about once every month. It has two drawbacks of which I am aware, when compared to a regular Mossberg 500.
It has a less expesive action slide that comes from a Mossberg subisidiary, Maverick Arms, and the long barrel does not accept screw in chokes. Instead the choke is fixed as a "Moderate". You can buy Mossberg's with interchangeable chokes, but that sale item doesn't have them.
Still, the Mossberg Big 5 deal isn't one to pass up without consideration if you want an HD gun that you can use to shoot some trap. In other areas of the country I believe other large retailers have similar deals (Dick's for one). WalMart in CA doesn't carry guns.
The Reminton 870 is a bit over your budget. I think the basic 870 can be found for $310 on sale at times.
Going with a used gun, as suggested, is not a bad idea. While I got a new 870 and I had to look for one, I found a used 28 inch vetn trib barrel with removable chokes for $85 in good condition.