I was on the border of Central/Sheung Wan by the Man Mo temple...it was 400 sq ft but beautifully done and I was paying $15k until I moved out last month. If you've always been a city dweller in smaller apartments (I lived in New York for years) then you will definitely get adjusted to a smaller space quite quickly, especially cuz you'll hardly be home due to work/socializing and all that.
However, apartments like that tend to be starter ones for the newcomer to HK...it does get a bit claustrophobic after a while, especially if you constantly have visitors staying with you. I managed to get through 1.5 years myself.
My old apartment may still be up for grabs (not sure though), if you're prepared to pay $15-$18k...just send me a PM if you are, and I'll ask my old landlord. If you're looking to spend less, consider checking with local agencies instead of the more expat-oriented ones...my friend just found himself an older 650 sq ft apartment with great space usage on Wing Lok Street in Sheung Wan for under $12k.