Click this link and you'll see the Samsung 4K 65" TV's WalMart has. One is under $500.
For the low-end LCD's, I like to get one that has good black levels. Usually those are a bit more than the absolute cheapest ones. You might have to do some investigating because typically the models Samsung makes for WalMart (or Best Buy) have different model numbers than the ones on their website. That means if WalMart doesn't tell you what the contrast is (the higher the better the black levels) then you might have to guess. Usually once you get over about $800 or so, the extra features you are paying for are stuff you probably don't care about.