Food is all over the map. If you want cheap, there's super dirt cheap as in 10 HKD for a bowl of noodles. Tough to estimate as I don't know what your normal inclinations towards dining are.

Octopus card is 150 HKD which includes 100 HKD in credits and 50 HKD card deposit (refundable if you return the card). If you're vacationing as a tourist, I'd probably say around 50 HKD/day. If you're working, about 20-30 HKD/day. Of course, this is purely MTR/bus as I'm sure you'll develop a taste for taxis.

As I've never rented rooms by the month before, I can't help you on that. asiatravel.com is a good place to get an idea of what a hotel will cost. You may be able to contact them for monthly rent prices.

Another factor to consider is your budget for vacationing to other locations from HKG. Lots of inexpensive side trips to other Asian destinations can easily be had.