just remember that cheap shouldn't be totally unhealthy! you don't wanna end up in debt because you were malnutritioned and ended up in the hospital!
firstly, get a multivitamin, take it every day- that'll help with overall health.
invest in a water filter, cause if you're drinking city water that stuff is nasty and will make your food taste wierd. i get water from my local organic grocery store, it's a BYO container and it's 39c a gallon- not bad
anyway, i say go with rice and pasta- hit up the Zataran's when they go on sale, they're cajun rice mixes and they're usually about 79c a box- one box is usually two meals and they're good! i like the beans and rice one- tastes good, and is filling. with plain rice you can add butter and soy sauce and you're good to go- invest in fresh veggies (which are usually cheap) to liven it up, and to get your nutrition. pasta is a no brainer- get some tomato sauce, add some garlic and some dried herbs (you can get those really cheap at Family Dollar, make sure you crush them up a little bit before adding them to the sauce to release their flavor). big pots of spagetti sauce last forever and can be frozen. also, hit up the cheapie groceries. i didn't see where you were from, but if you know of Family Dollar stores they sell cereal/poptarts/ boxed goods cheap and personally i think their offbrand pop tarts are better than the real thing! mac and cheese with hot dogs is also good. another thing i would recommend is save up some money, like take your pocket change and put it in a jar or something, then every week or two treat yourself to a "high class" meal, like get some ground beef and make tacos or invest in going out somewhere and just getting an appetizer. it's small things like that which will let you know you love yourself even when faced with being a poor poor poor college student
