£100 a day! :eek:

Give over!
I just wrote a long reply and accidentally deleted it, so I will start again.
September is a good month to go as the hotels are coming off the summer season and have good deals. Be prepared to leave this until the last minute as the prices will invariably be better. Hotels.com and hotelclub.net are other alternatives for late deals.
Nobody eats in hotels, except for breakfasts that come in the price of the room or businessmen. When we lived in Macau, we used to spend 2/3 nights at a time in HK every fortnight and would usually get by on £50-70 per day including all meals, transport and hotel. That's two people. Obviosuly if you are on your own, there wouldn't be to much difference on the hotels as you would be in a single room supplement.
As for hotels, Kowloon has a cheaper range, although a lot of the affordable ones are not well served by transport links. HK Island has more near good transport links. I can recommend the YMCA by Causeway Bay, which you can normally get a room in September for £30-40 per night, sometimes less if you are lucky. It is comfy, clean, friendly and caters more for businessmen rather than grotty students. There are others - look on the websites and give us an idea of what you like the look of and I will let you know what I know. Steer clear of most places in Wan Chai, unless you like being dead close to the pubs and the prossies.
Eating out - we used to pay £25 on average for a meal for two with drinks and dessert. So do the maths, maybe add a little bit for being a year in advance - say £16 per meal per person. There are a massive range of eateries from Italian, Pizza, pub grub (irish bars too), local and so on. What we used to do is eat at the hotel in the morning, go to the supermarket or bakery for picnic bits for lunch and eat out in the evening.
Transport - get an Octopus card (I think that's what they're called!) If you are travelling to Lantau, New Territories, Stanley, tube and bus a lot, it really saves you money - even if you are there for a week.
Not sure what you want to spend money on so maybe think of that on a different budget.
For day-to-day spending, you can get by on £50-70 max per day for hotel, food, meal, drinks, transport, etc. So over ten days you would be looking at £700 tops.
Days out and things - stick to transport (use your Octopus card!) and visit places, walk around gardens and the bustling areas of the cities, there is so much to do free and/or on the cheap. I could give you a long list of places should you wish.
Let me know if you want other info. We are planning on going back out there next summer for a fortnight and I can't wait. It is my favourite place in the world.