dried beans/rice are stupid cheap and safe. just wash and rinse everything a few times, which you're supposed to anyway even with good beans/rice to remove dust and whatnot to improve the texture.

if you're real suspicious, stick to whole, intact plant material... things you can wash the outside of, especially things you can easily peel like potatoes and apples. most bullshit doesn't penetrate protective, outer layers. kinda sucks, because i like potato and apple skins and the insoluable fiber is good for you. of course, this means leafy greens are suspect compared to like, a turnip or a beet because of the high surface area. but just wash em.

processed and already cut/ground items from shady packers are the vector for weird shit from unsanitary practices. places not training employees, bosses half-assing shutdown/cleaning shifts, skimping on the cleaning materials or the introduction of contaminants... because how are you gonna know when the thing is already sliced, diced, canned/bagged and ready to microwave? but you can look at a potato or a pepper and see if it's been damaged before you buy it.

basically the cheap/nutritious path means processing food at home and working with basic ingredients, which means you're sacrificing your time/labor and having to learn how to make things. and depending on what you buy, having to go to the store more frequently because a lot of produce doesn't stay fresh long, even in the fridge. which isn't all bad, as food can be an expressive art form and a sharing of oneself with others. but sometimes you just want to put in a cheap oven pizza and let it happen. so, it's more of a process than an orthodoxy.