Just to add to the singapore issue, my experience in buying Rolex in singapore (just two months ago) was that after the discount given by the singaporen AD the price is normally quite competitive to Malaysia's AD price, although not necessarily lower. It all depends on the exchange rate. When i went there it was 2.51 so in my scenario it was slightly more expensive. However, Add on top of that you get back some GST from the government in the airport (make sure you fly there and don't drive there as you can only claim GST if you fly). After the GST claim it was slightly cheaper.
Then you have to also consider bank charges, credit card charges and the exchange rate the bank use. All in all if i had bought the Rolex in Malaysia i would have saved RM500 after all calculations done. If you specifically flew in to Singapore to buy the watch then offcourse you should include that charge in. For singapore purchases the exchange rate is king for your decision. If you buy at a low exchange rate, lets say 2.4, it would make a world of difference (offcourse dependent also on how expensive is your watch).