First, suggestions (aka what I ate):
Oatmeal, peanut butter sandwiches (especially with bananas or home-made jam), and the cheapest organ meat you can find. Ground beef heart can make a decent meatloaf, in a pinch.
(I got that from the "Meats Laboratory" at Chico, which is a slaughterhouse run by the university to train students.)
Yes, it can be a problem.
I ended up spending around $10/week, for about the cheapest food I could get.
Most of the money I used to pay for a university education was from work I did in the meanwhile for $8.15/hr, so it's not impossible to work your way through.
The biggest thing is to find a place to stay that's close enough and cheap enough. I was working at the university farm and staying there as well, for ~$150/month. If I'd had to pay the $500+ that would be more typical, I don't think it would have worked.