I just checked my local grocery store website. The cheapest wine they have is $4 for 750 mL, and $5 off if you spend $15. So you could buy 4 bottles for $12. That's still $3/L.

But ah, yeah, your point still stands. Wine can be REALLY cheap.

Though back when I worked retail, I only ever had one guy that was always buying lots of wine. The customers that came in frequently to buy alcohol typically bought 12- or 24-packs of cheap beer, though the homeless would buy 40 oz Olde English 800 or the 24 oz Steel Reserve cans.

FWIW, I live in Oregon where hard liquor has to be bought from licensed liquor stores, not grocery stores.