Hey Waldo-
Pork is not graded by the USDA like beef is. It is inspected by the USDA (domestically raised anyway, not sure about imported).
As for the difference in price, could be any number of reasons. Seperate from the grading issue, both pork and beef are marketed under different "brands" for lack of a better term. That could be one reason.
Another reason might be that the Wal-Mart butt has had "solution added", sometimes refered to as "tumbled" meat. What this means is that the meat has had a salt/water solution added. In theory, the solution is supposed to make the meat more juicy. Whether that assertion is true or not, it does mean that you are partially paying for saltwater. The Sam's butts I purchase are not tumbled, while the meats I see at Wal-mart generally are. Seaching the forum for posts on "tumbled" meats should produce a lot of posts if you want more info.
Was Wal-Mart running a special? If so, it's possible that they lowered the price to entice customers into the store for the butt. They might make little or nothing on the pork, but they figure you'll stick around to buy higher margin items. It's the same thing almost all grocery stores do at Thanksgiving when they sell their turkeys for dirt cheap.
You were on the right track about grading in one respect, however. In my area, Wal-Mart sells almost exclusively Select graded beef, and my Sam's sells almost exclusively Choice graded.
If you are wondering which one to buy, as inexpensive as pork butt is, I try both. Cook them both up the same, and see which one you like better. To find out if one is really a better deal, you might also compare yields on the two.
Hope I gave you at least a little useful info in all that typing!
Matt