Any time a store does something to a fruit or vegetable before putting it out for sale, you’ll pay up to 40 percent more to buy it. So slice your own melon, cut up your own carrots, and make your own salads. It won’t take as long as you think, and the prep work is totally worth the money. Pro tip: Set aside half an hour after grocery shopping to wash, chop and store enough produce for the next few days.