I recently started using one of those budgeting apps to check how much i was spending, and i'm averaging about HK$ 11-12k each month (gonna break the 10k barrier soon
).
This doesn't include rent and tax, but includes all bills, food, transport, going out etc. I am living on my own, so you will expect to pay double on some aspects such as food and transport, whereas some bills may be shared. I also own my place, so also include management fees, government rent, land rates etc. in bills, which you probably wont have.
i would expect you can get a nice place in south lantau for less than 35k but, as a note, food takes up over 4k of my budget (i normally eat out for lunch and maybe 2 or 3 dinners a week) for just 1 person.
It is possible to eat conservatively. most local restaurants have dinner sets for about $50 each, and in cooked food markets, there is usually place where you get a rice tray with 2 or 3 items for about $40. This is great if you cannot read Chinese as you can just point to what you want instead of going to more expensive restaurants for the English menus.