It depends on where you shop. The prices at Niks Thailand (the Nikon showroom) on Silom Road are on par with prices in Singapore. The best prices I've found are in a few of the shops at Fortune Town, at the Phra Ram 9 subway stop. Gray market prices there are on par with the lowest prices I've seen in HK. And a couple of the shops give good discounts on lenses that carry the Niks Thailand international warranty.
The shopkeepers I've bought lenses from at Fortune Town are patient about letting you test the lens on your camera before you commit to buy, whereas you might not get that same level of service from some of the busier stores at MBK and Panthip Plaza. The only problem is that the smaller shops don't carry a lot of stock and therefore may not have the particular lens you're looking for.
