Estate, Rio, Nobelsport, and Centurion have good bulk deals online too. Much, much better than $1 a round which is about as good as you can do at Walmart on defensive/hunting 12ga.
http://www.ammunitiontogo.com/product_info.php/pName/250rds-12-gauge-nobelsport-le-2-34-12-pellet-00-buckshot/cName/12-gauge-2-34-buckshotThe Saiga 12 folks who are always looking for cheap ways to feed their big box magazines and drums seem pretty happy with some of these offerings.
And Nobelsport also has a solid .65 caliber ball option that's very cheap too.
http://www.ammunitiontogo.com/product_info.php/cPath/120_259_206/products_id/5109IMO, the only thing less expensive than Nobelsport when it comes to buckshot or solids is rolling your own loads from components and once-fired hulls. Primers, wads, powder, and scrap lead... if you can find the once fired hulls for free, or you're reusing your own, then it gets down to around $.16 a shell or so. Also note, they need not be premium "high brass" hulls. It's more of a marketing affectation these days. The plastic of the hull, and the bolt/chamber of the shotgun really is what's providing all the support anyway. Low brass will fire a slug or buckshot load just as well as high brass will. It's a hold over from the old days of paper shells, and wound Damascus steel shotguns when the brass actually did provide some modicum of extra support. Now it's just tradition. And a place for the extractor to grab onto.
They never caught on but Activ even produced an all plastic shotshell for several years. (It did have a metal washer molded into the base for primer support, but that's it...) They went out of business, but their shells worked just as well as anyone else's and were just as reloadable too. As ammo costs keep going up, I expect some of the makers to get back into all plastic shells eventually. IMO, not really a bad idea. It's just that much less metal to corrode IMO.
Anyway... at reloading prices, it's VERY EASY to recoup the costs of a melter pot, a mold, and a Lee Load-All II (around $120, give or take...) in just a few hundred rounds. And in general, shotshell reloading is easier and somewhat more forgiving a process than centerfire brass cartridge reloading in certain ways. And you may have some VERY LUCRATIVE equipment and skills come SHTF, or even just a prolonged economic downturn. And it's much cheaper to stockpile components than it is finished ammunition. And easier to spread out the costs too. Primers one month, hulls another, scrap lead wherever you find it, powder when it's on sale, wads are always cheap... etc.