i grew up in hong kong but have also spent a number of years in the states. i think it's unfair to compare hong kong vs new york rent prices when you're only looking at the prime real estates on hong kong island. if that's the case then we should only be looking at the upper west side of manhattan.
i live in yau ma tei, which i suppose is comparable to somewhere like williamsborough, where i pay HK$3500 a month for an 800+ sq flat.
if you eat like the locals do, there are plenty of meals (ie with drink + soup/dessert) within the HK$30-40 range.
but of course if you're going to only look at things along the lines of a typical expat lifestyle, then costs can add up pretty darn quick.