Unagi don is 671 calories at Sukiya for around 1,000 yen, which is the cheap end. The eel itself is fatty, about 293 calories per 100g as kabayaki, so an 80g piece is 234. The rice is 260g and about 405. The tare is sugar and soy, around 30 calories for the drizzle. Compared with a gyudon, you pay double and get about 40 more calories and 10g more protein, but the eel has omega-3 fat rather than beef tallow.
The specialist unaju is a different portion. A proper eel shop lays a whole 150g fillet across the box, and calorie.slism.jp puts that at 879 calories with 42g of protein. Those meals cost 3,500 to 5,000 yen, so people eat them once a year in July. The calories are high but the protein is real, and the fat in eel is largely unsaturated. As a once-a-year meal it is one of the more honest 900 calories in Japanese food.
The extras are small here. Kimosui, the clear soup with eel liver, is about 20 calories. Pickles are 5. Sansho pepper is nothing. So the number you order is the number you eat, unless you go 'omori' rice at Sukiya, which takes the bowl to 817 published. The eel does not change; only the rice does.