Balut is an egg, so it counts like an egg, just a big one. A duck egg is about 70g of edible content versus 50g for a chicken egg, and duck yolks are richer. The figure that circulates from Philippine sources is 188 calories and 14g of crude protein per balut, and that lines up with USDA duck egg data at about 185 calories per 70g. The embryo adds some protein and calcium (about 100mg) but does not change the calories much.
As a snack it is one of the best protein deals on the street. 190 calories with 14g of protein for 20 to 25 pesos beats almost everything else the same vendor sells. Chicharon, which balut vendors often carry as well, is 150 calories for a small bag with 8g of protein and a lot more salt. The yolk carries the fat, about 14g, half of it saturated, and about 600mg of cholesterol, which is the thing to be aware of if you eat 2 or 3 a night.
Penoy is the unfertilised version and is basically a boiled duck egg at about 180 calories. Balut sa puti, the younger 14 to 16 day egg with a white membrane and less embryo, is about the same. The only way balut gets expensive is the habit of eating it with 2 sticks of pork BBQ and a beer, which turns a 190 calorie snack into 600.