Zaru soba is about as light as a full noodle meal gets: 362 calories, 2g of fat, and 16g of protein because buckwheat has more protein than wheat. The dipping sauce (tsuyu) is about 40 calories for the small cup, and the nori and wasabi are nothing. A plate at a standing shop costs around 400 yen, and at a proper soba restaurant 900 to 1,200.
The catch is that it is not very filling for the calories. A 362 calorie plate is 71g of carbohydrate and almost nothing else, so you are hungry again in two hours. That is why soba shops sell the 'ten zaru' set with a plate of tempura, which is 532 calories per calorie.slism.jp, or the 'ni-mai mori' double portion at about 700. Most people do not stop at one plate.
The sobayu at the end is worth knowing about. It is the cloudy water the noodles were boiled in, poured into your leftover tsuyu and drunk like a soup. It is only about 15 to 20 calories and it turns a plate of noodles into something that feels like a finished meal. Drink that instead of ordering a second plate.