A 1kg mud crab gives about 250g of meat once you have fought through the shell, so half a crab is about 120g of meat at roughly 100 calories. Crab meat is lean, about 85 calories per 100g. What you are eating on top of that is the coating. Black pepper crab is fried in a good amount of butter, about 20g per half crab at 145 calories, with oyster sauce, soy and a little sugar for another 60. That is 344 for half a crab. Most of the sauce stays on the shell, which is one of the quiet advantages of this dish.
Chilli crab is about 450 for the same half crab, because the gravy is thickened with egg and has a lot of sugar and tomato sauce, and it is made to be eaten by the spoon with fried mantou. Black pepper crab has no gravy to mop up. The pepper sauce is dry and clings to the shell, and after you have eaten the meat there is nothing left on the plate to drink. That is why it lands about 100 calories lower.
The mantou is the trap for both crabs. A fried mantou bun is about 130 calories and the basket comes with six or eight. Two fried mantou with the crab takes 344 to about 600. The steamed ones are about 80 each. Order steamed, and eat the crab with a plate of stir fried vegetables at about 150 instead of rice, and the whole meal for one person is around 500.