I eat alot of fast food because its cheap, but I'm completely cutting it out.
What are some things that are healthy (and good) but very cheap?

check the sale flyers for your local grocery stores...
find a local farmer's market for fresh fruits and veggies.

One thing that I find very very cheap, endlessly customizable, tasty, and filling is beans with rice. My two favorite ways to do this are: black beans cooked through with spices, then take about 2/3 cup or so and put it over a big spoonful of rice that has been cooked with small diced tomatoes, onions, cayenne, paprika, and whatever other spices you like (dirty rice). I usually use white rice but I'm assuming you can use other kinds as well. I also like Cajun style red beans and rice, where the red beans are simmered overnight usually and often cooked with some meat. This can be done in a crock pot and is usually make really spicy. Then, again just ladle it over top some rice and you have a pretty nutritious meal that will fill you up. Cutting an avocado up over the top of the black beans or mixing corn in with them gives you some vegetables too. I like sauteed squash alongside either of these dishes. I also like green beans just barely cooked so there is still bite to them, with lemon juice, salt and coarse black pepper on them. The green beans are better next to the red beans and rice, but will work with the black beans.