Every morning this gorgeous piazza hosts a food market. I am not going to be able to do it justice. It is full of tomatoes, chillies, fruit, garlic, onions. But not as we know them. The bland unripe unseasonal rubbish that you find on the shelves of British supermarkets bears absolutely no relation to the stuff you can buy (for pennies) in Campo dei Fiori.
For a cheap lunch just come here early, buy a few tomatoes and some foccaccia, stuff them in your bag, and eat them at a Bernini fountain in nearby Navonna while laughing at the fessi (gullible ones) who have paid through the nose to eat tourist junk at the inauthentic cafes.
At Campo dei Fiori of course. Just north of the Jewish ghetto along the east bank of the Tiber
Updates
The fruit and vegetables in Campo de Fiori are absolutely fantastic - I used to walk there everyday when I was in Rome and most of the stuff I bought didn't survive my walk back to the apartment.
Is this your business?