I am getting really surprised when someone says "Ohh, Hongkong is so expensive city, even the food. There is nothing to eat". Come on, you just need to find the right places!  Hongkong is full of the small cafes, ramen shops, sushi bars and dim sum restaurants, quite affordable. Most of the are run by the chinese family business, except of "Cafe de Coral".  The last one is our first stop.

1. Cafe de Coral
Address : many, it's everywhere in Hongkong. Please refer to the website to find the closest to your area
http://www.cafedecoralfastfood.com/shops.php?lang=en
Visual : look for the yellow sign with the red logo on it.

What to eat : good breakfast sets (the chicken with maple sauce is gorgeous), dinners (steaks, hotpot, BBQ, roasted meat etc.).  Lunch is so-so...
Service : self.  Pick up the "wall-menu", remember the code then say it at the cashier. Get the table and wait until you see the code running on the panel. Go to pickup your food at the counter.
Average bill : 25-50 HKD (4-7 USD) per person


2. Itamae Sushi.
 Address : many in Hongkong, you may try one of these
-  22-26, Wai Tsuen Road, Hong Kong, China
- Shop Nos 1-3, 24-26 East Point Road, Causeway Bay
- Shop 1-8,1/F,Jialefang, No.56 Dengdashi Street

Visual : look for the brown sign with red hieroglyphs
What to eat: best fresh and tasty sushi, sashimi, miso etc. 
Service : menu or conveyer. Coming closer to 23-24h you may have to run into the electronic que, as this place is quite popular among locals.
Average bill:  100-150 HKD (15-20 USD) per person


3. Urawa Japanese Restaurant
Address: they have three currently
- Basement & G/F, Tung Hip Commercial building, 244-252 Des Voeux Road Central, Sheung Wan
- Shop A No.4 Landale Street, Wanchai, Hongkong
- 3/F, Lik Sang Plaza, Tsuen Wan, N.T.
Visual :look for the brown sign with red hieroglyphs

What to eat : sushi, sashimi, rolls, soups, oysters etc.
Service: menu or buffet (the last one is awesome)
Average bill: ~200 HKD (30 USD) per person (you will be really full, believe me)


4. Good start cafe
Address : Fa Yuen Street, Mong Kok, 110 Ground Floor.  They may be more, but I do not know the exact locations.

Visual : look for the red sign with white hieroglyphs
What to eat : dim sums, many and tasty. Also the traditional chinese soup and some side dishes.
Service : menu
Average bill : 40-50 HKD (6-7 USD) per person


5. Fuk Yuen Hotpot Seafood Restaurant
Address : 2/F,Yau Shing Commercial Centre, 51 Sai Yeung Choi Street, Mongkok
Visual : quite hard to notice.  Just follow Sai Yeung Street until you see "Cafe de Coral" sign. The Fuk Yuen is in the same building.
What to eat: lot'sof traditional chinese food (roasted duck, goose, black chicken soup) and excellent seafood.
Service : menu
Average bill : 150-250 per person (depends on what you take, cause they have good lobsters)


6. Ajisen Ramen
Address : many in Hongkong, please refer to the website to find the nearest to your area http://www.ajisen.com.hk/web/restaurant02.html

Visual : look for the cute little girl with ramen bowl sign :) 
What to eat : salads, ramen and set meals
Service : menu
Average bill : 50-60 HKD (7-9 USD) per person
I could continue writing about all these cosy places, like Yoshinoya, Lau shan, and so on.  But I recommend you to go and try instead.  Find the new spots and share them!