The ladies who have replied all have great suggestions. I would also suggest that if you can't afford healthy foods or don't have access to farmer's markets or a garden, just make sure that you are sticking to portion control. We all know that we can't eat entirely health 100% of the time, but when we can't, sticking to proper portion sizes will help. I.e. - if you're having a peanut butter and jelly sandwich, make sure you are only using 1 tablespoon of PB and 1 teaspoon of jelly, and try to swap white bread for whole wheat. Making some simple substitutions and measuring out your portions can help keep you on track.
I would also suggest checking into a Sam's Club or Costco if you have one in your area. A one-year membership is usually around $30-40, but it can save you alot. They often have lots of fruits and veggies at good prices all year. Also see if you have a Gordon Food Service (GFS) which sells in bulk without having to buy a membership. Some other cheap healthy foods are: quick oats with splenda or stevia (the kind you boil for a minute on the stove), frozen veggies, frozen fruit (make sure no added sugar), a huge bag (like 10-12 breasts) of individually frozen boneless, skinless chicken breasts are only about $13 at Sam's, a big bag of apples and oranges is only about $3-4, sweet potatoes and red potatoes, a big tub of plain or vanilla low-sugar, low-fat yogurt for making smoothies with frozen fruit (cheaper if you buy a large tub of one flavor rather than little individual containers), and I know Wal-Mart has bags of frozen tilapia and salmon fillets (about 4 fillets to a bag) for only $3-4.