As a kitchen-illiterate college student who can't stand cafeteria food or afford delivery, I live off of Soylent, Coffiest, and Cacao (the newest Soylent flavor). They're relatively cheap ($2.69-$3.25/bottle, depending on which flavor & how you order) compared to anything but fast food. Carrying them up from the mailroom is a pain, but they taste great & fill me right up.
Definitely try the different flavors - I have friends who originally hated Soylent Original sneak into my room and steal Cacao out of my fridge.
Like many people in San Francisco, I don't have a car so grocery shopping is very time-consuming and/or difficult to carry home. So instead, I use Instacart ~once a week. You can order from about a dozen grocery stores but if you want the best deal, order from Costco or Cash and Carry. Both offer bulk foods at 2-4x cheaper prices than Whole Foods or Safeway.
My thrifty pro tip: rotisserie chicken ($10) + giant bag of spinach ($8) + salad dressing ($4) = $22, which serves about 4 meals.