That depends. A friend of mine works at a winery and they sometimes make deals with large supermarket chains to buy a substantial amount of the harvest at a low price. Gives them a predictable income with no marketing costs attached since supermarkets will commit to buy any amount they want at a fixed price months ahead. The supermarkets sell under a different label (they wouldn't even tell me which one) but it's the same wine at 30-50% the price. Supermarkets are incredibly efficient in logistics and selling and sell the wine at very little above the price they bought it for whereas local stores/vineyards will often have to sell at 100% mark-up.

So just because a wine is cheap doesn't mean it's bad. There are a few supermarket wines I found (in Europe, esp at Aldi/Lidl) that were incredible good for the price. In countries with low alcohol taxes (e.g. Germany), you can sometimes get a very good wine for as little as €3/$3.50 a bottle. It's rare, though, since they often sell out quickly once word spreads that this is a decent wine.