We ate out a good amount, and drank wine often, so while we spend more on food on a Whole30 we save on those things. I now make my own kombucha and we bought a SodaStream machine which we use to make sparkling water.
Cheaper options we buy often:
Canned salmon - make salmon cakes or just toss with mayo for salmon salad.
Wild sardines in water or tomato sauce
Make Bone Broth yourself from the leftover bones from chicken, beef, pork bones. Talk about stretching your dollar!
Buy whatever is on sale, fresh or frozen. Fresh/local winter squash, etc can be found for 99c - 1.99 a lb often.
Buy veggies that are most nutrient dense. Sweet potatoes fill you up more than cucumbers or jicama and pack way more nutrition.
Buy local or in-season veggies. Produce from far away will cost more (for you, and for the environment/air quality).
Skip the nuts, they make grass fed beef look cheap in comparison.
Buy a big tub of coconut oil on Amazon subscribe & save (discount!) or other internet source if you don't have a Costco/Sam's Club membership.
Canned olives (read your labels) are another option.