I once saw a study saying that eating organic vegetables rather than the cheaper conventionally grown ones had no measurable health effects so you should be fine eating the cheaper conventionally grown ones. They're still as good for you. Also, sometimes you can get lower-graded vegetables cheaper. The grading is purely cosmetic, the vegetables themselves are just as tasty and healthy.

As for staples like pasta, rice and flour the cheap versions are completely fine. Higher quality pasta is better at soaking up sauce, higher quality rice has better aroma and larger grain size but the cheap stuff is just as healthy and nutritious.