I don't consider some of my changes to be "gluten free" changes that cost money. So I buy Chex or Pebbles, I did occasionally before too. It just is more frequent now since I'm not buying so many other cereals and doesn't cost more than cereal did before.
Where I used to eat, by myself, probably 2 loaves of bread a week, I now go through 1 loaf of gluten free bread a month. Frequently less. A great and cheap replacement is corn tortillas. Grilled cheese? Quesadilla! It is exactly the same thing, only with a flat bread made of corn. Tortillas aren't some magical food that is made to be gluten free, they're just born that way because that is what they are. A local store sells pasta in bulk and offers a 10% discount if you buy it in the boxes. I just buy that way so people haven't been touching it and crap, it's all wrapped up in it's box still when I bring it home and even cheaper than regular bulk.
Mostly I watch for sales and don't buy "gluten free" food. The food that is good for you is naturally gluten free anyway. Meats, fruits, vegetables. In cans and frozen you'll want to still read labels to make sure of sauces or seasons, but if we just stick to the basics it isn't expensive at all.