I think that one particular Mossberg model stands out for a good bang for buck. The Mossberg 500 Persuader, 12 gauge, 6-shot, 18.5" barrel. They can be had for approx $300 depending on where you buy from.
The reason I think it is a good bang-for-buck is the following. The finish of this particular model is a true blued finish, not the more fragile "matte blue" that most entry-level shotguns have these days.
Also, the slide action assembly on this model has a true tube assembly that the forend attaches to. A lot of the entry level Mossbergs (and all Mavericks I believe) have a pinned forend, which means the slides are pinned directly to the forend. This means you cannot easily change out the forend if you wanted to. You would have to buy extra parts if you decided to go that route. Also, the pinned forends have a lot more "slop" to them. Having the tube assembly makes the action tighter and less sloppy, also it is just a matter of unscrewing the compression nut for changing out forends.
Here is the model (#50411). If you look, you can see the better finish and if you look closely, you can see the nut in front of the forend.
For reference, here is a Maverick 88 with the pinned forend. If you look closely, you can see there is no nut in front of the forend. Also, look to the rear of the forend and you'll see the pin holding the slide to the forend.
The only reason I bring this up is, I bought a Mossberg 500 from Cabelas. It was a combo deal and while being a great gun that I never had a problem with function wise, the matte finish scratches easily. Also, I wanted to change out the forend for a Hogue rubberized forend. That change over cost me a lot more then I originally thought, because I had the pinned forend and had to buy a tube assembly (aproxx $40 for the tube action slide assembly alone).