Yeah, all of those are single serving prices that you're looking at, and only the onion (and I guess lettuce if it's not romaine) is actually a vegetable. Potatoes are starches, the others are fruits (with higher sugar). They don't list much in the way of vegetable prices.
But, even given that; I can buy a head of lettuce for $1.40. Okay. Or, I could buy an 8 pack of little debbie snack cakes for the same amount. 5 servings of lettuce that has nearly no nutritional substance, and that I have to combine with something else anyway to make it palatable...or 8 servings of convenient, enjoyed food.
If I wanted crunch, I can get a giant bag of tortilla chips for $1, too. Why would I buy a head of lettuce for 40% more, when it takes more work, and more ingredients inflating the price even higher?