WANT to buy a book, roll of film, some batteries, or simply a packet of anti-cold tablets? Then avoid Kai Tak Airport where, as we reveal today, you could find yourself paying up to 115 per cent more for exactly the same product in town. Greed is not entirely to blame for Kai Tak shopkeepers elevating overpricing to a fine art. With rents up to three times as much as those in some of Hong Kong's most prestigious shopping centres, most shops are left with little choice but to charge over the odds. As a Civil Aviation Authority spokesman admitted, Kai Tak is a ''retailing gold mine'' and the authority reaps the benefits of that by renting out the airport's scarce space to the highest bidder. That may not matter to those who live here and have enough sense to shop elsewhere. But it is of concern when it affects the territory's tourist industry and its international image. Ripped-off on arrival, and ripped-off again when they leave, many tourists will depart with negative memories of the territory that are unlikely to make them want to return. Fortunately there is a solution at hand. The Provisional Airport Authority pledges that the future Chek Lap Kok airport will have much more space for shops and charge prices similar to those found elsewhere in the territory. That is, of course, assuming the authorities abandon their sky-high airport space rental policy for one that is down to earth.