It depends what you call "relatively cheap"...
The cheapest of cheap, yet respectable, are the Rem 870 Express, Mossberg, aka, Walmart special, $250-350 shotguns. Nothing wrong with these shotguns.
Personally, I'm a huge fan of the Benelli Supernova. I have 5 of them for different purposes, as well as different barrels for a few of them (or really for all of them?). Around here, they run $450ish, and new barrels are anything from $150-400, depending what you want. For me, having $600 in a shotgun is "relatively cheap", since I can't say I have a rifle that's that "cheap", and the shotty's cover WAY more territory than any single rifle I have.
The great part, for me, with the Supernova's is the versatility. They're incredibly lightweight, yet still have very low recoil, even with 3.5" mags (ok, RELATIVELY low recoil), and the modular system makes cleaning a breeze (as well as upgrades, modifications, gunsmithing).