To me, HK$ 250 is the absolute maximum for dinner.
You could have great Japanese seafood buffet for that money.
I don't like paying anything over $50 for a single dinner.
problem with the thresholds people put in this thread is that it's extremely subjective. For someone who loves to eat, the price they're willing to pay will most likely be higher than the person who simply eats for sustenance.
Sushi (btw which buffet are you referring to?)
For Japanese buffet (mind you, not just sushi!) which is under $250, try the following:
DaiKiYa @ Central & TST
Japanese Buffet DaiKiYa Japanese Restaurant's Overview - Hong Kong Restaurants Guide HK Restaurant - OpenRice in English
(TST is more spacious, my personal favorite)
Kichi Jyu @ Central & MK
日本菜 Japanese 自助餐/放題 Buffet 吉壽日本料理 Kichi Jyu Japanese Restaurant 的相片 - 香港餐廳 - Hong Kong Restaurants Guide HK Restaurant