It's a lot cheaper than eating out, and (often) a bit cheaper than cooking yourself. On top of that, it's a lot easier to just shake a bottle and eat than to cook.
Soylent actually competes pretty well on price too! The calories on those recipes range from 140 to 386 per $1 serving. That's $5-14 for 2000 calories, while the new Soylent formula is $12. (Slightly more if you don't subscribe, slightly less if you get the powder formula instead of pre-mixed.)
Plus, the recipe cost assumptions must be pretty generous to get to $1. One of the recipes calls for 1 pound of ground beef and makes 6 servings. The flyer for my local discount grocery has the ground beef at $4.50, but the nicer grocery nearer to me is already over $6 just for the meat, let alone two cans of tomato sauce, some pasta, some carrots, shredded cheese... I'll bet in a real-world test of those recipes, Soylent would beat the average cost.
http://www.watchusthrive.org/Libraries/Resources_Documents/B...