I've found that dark leafy greens can be pretty cheap (collards, kale, mustard, dandelion... whatever's on sale). I can't always afford organic, so the fact that I can get ridiculous vitamin/mineral content for less than $1/lb makes me happy. Organic carrots also tend to be cheap. Do you by chance have an international grocery nearby? I get veggies (albeit conventionally-grown) at good prices when budget's tight.