If you really need to be in Hong Kong and can't afford a good hotel, CASA Hotel is ok for as long as you can stand a very small, dark room with a window but no view at a price about four times what it would be in most countries. This place reminds me of a small RV and has a wet bath too (shower is on wall and gets everything in bathroom wet and not much room for a shower at that). Room was clean enough and had enough amenities though I moved as much as possible out of the way into drawers or wherever so there was a little space for my stuff. Single bed was small and hard, against the wall. Extra pillow and blanket in closet. Air conditioner worked ok. There was a note about water getting hot 10 minutes after you enter the room and there would be enough for a second shower after a while - I took a quick shower in the morning and there was enough hot water but didn't take a shower in the evening, as I usually do in a climate like this, to avoid getting the bathroom wet. Wireless internet worked ok after entering password but that password stuff is always a nuisance.
You can get a far nicer hotel in Shenzhen (mainland China, just across the border) for the money, better food than in Hong Kong (I tried the expensive Hong Kong Sheraton (down Nathan Road) buffet breakfast and wasn't impressed except for the selection of fruit), and a more pleasant environment in Shenzhen. For a single room, take the ferry to Nan Hai Hotel in Shekou for a much better hotel at half the price.