Sure, 500 would be a lot for me, too, if it were a normal month. But then again I walk to work, almost never take taxis anywhere, know all the cheapest local eateries, shop only for day-to-day groceries and don't visit tourist attractions.
Flag-fall on taxis is $18 these days, isn't it? And if you don't know your way around, are going from tourist spot to tourist spot, or otherwise prefer the convenience of being driven around HK, then taxis alone could add $100 or more per day to your bill. If you were to stay in the same hotel as my brother's company chose when they last came to HK, then it's $200+ in a taxi just to reach TST.
It's a couple of hundred to go to Ocean Park. About $500 (I think) to go out on a boat to see the pink dolphins. Then there's the Ngong Ping 360 at around $160 (including entry to the Buddha). $10 for the galleries at the Arts Centre represents great value for money, however performances of Handel's Messiah or Karl Lo's piano recital will set you back between $50 and $200. The Star Ferry Harbour Tour is about $50, I think, and the Peak Tram is around $33 (return), with the Sky Terrace adding a few dollars on top of that.
These in themselves aren't exactly 'big ticket' expenses, however they do add costs to a tourist itinerary which an HK local wouldn't normally meet. Then, of course, there are the shopping items. Who comes to HK and doesn't visit a tailor? A few hundred for a shirt. A few thousand for a suit. The night markets are great for finding bargains, however bags full of gifts for family and friends soon start to add up.
In coming up with a $500 per day figure, I tried to estimate how much I might spend in a month as a tourist and then break that down to a daily average. It could certainly be done cheaper. No doubt about that. It could also, however, end up costing quite a bit more.