Transportation is cheap if you buy an octopus card which allows travel on everything - very easy and a much cheaper option http://www.octopus.com.hk/get-your-octopus/en/index.html
Food can be cheap if you eat locally. Dont eat on the main streets but chose a place off the beaten track which is frequented by locals. The menus wont be in English but usually have a picture on them. Street food is great too - but make sure its freshly cooked.
HK is definitely not the cheapest place but it is an amazing city. about 8 years ago
MRT isn't expensive at all. The system is quite extensive, therefore it will get you to most of the key places.
You can have a decent meal, tasty and enough to fill the stomach for HK$50 per head. Best to eat at the Hong Kong style cafe, inexpensive, quick and decent. about 8 years ago
I find the HK mtr very efficient and cheapest way to get around hong kong. There are a lot of street food around. If your not into that, one option is cafe de coral, they have a lot of selection and are almost always near mtr exits. Prices vary from 20-40HKD. Load up on your octopus from the airport. You can either get the airport express with 3day unlimited mtr use or the regular card which you can use on both bus and mtr, but the load varies depending on where you wanna go. If your a group of 4, get a cab from the airport to your hotel, this would be cheaper than taking the mtr and more convenient. about 8 years ago