This is probably one of the biggest misconceptions. I have found that eating clean can be less expensive than eating crap. I survived college eating the following, on a part time retail job:
My favorite protein sources that are inexpensive:
My favorite Carb sources that are inexpensive:
Rice (white or brown)
Whole Wheat pasta
My favorite fat sources that are inexpensive:
Toss in lots of veggies and you're golden. If you get a crock pot and rice cooker you can some of the best meals.