I cook 100% plant based. For a while I was fully vegetarian, but now I make "exceptions" if I'm somewhere like a Brazilian steakhouse with friends. But putting aside eating out, vegetarian cooking is 1) cheap, 2) easy, and 3) delicious once you figure out what you're doing. If you want to go full frugal mode, then eating a variety of beans, grains, and vegetables will go a long way. The trick is to build out your spice cabinet and settle on a primary "base" that you can use for most meals. I put onions, celery, carrots, and garlic in most things.
Then there's the whole ethical side of plant based food which people LOVE hearing about constantly from veg-heads.