Sweet potatoes can be considered a superfood. They are one of the healthiest and cheapest vegetables on the planet. (And one day, perhaps, even off the planet, as NASA has chosen the sweet potato for space missions.) A study out of the University of Washington aimed to identify which vegetables provided the most nutrients per dollar.The healthiest foods, like dark green leafy vegetables, may also be the cheapest, and the highest nutrient-rich food scores per dollar were obtained for sweet potatoes.