Also depends on the brand, in my experience. Prestige brands popular among Asian high-flyers, such as Rolex, aren't really cheaper compared to other places (my benchmarks are ZRH and LHR), but less widely-marketed manufacturers like IWC can be attractively-priced. As Gaucho100K pointed out, it never hurts to negotiate. (Negotiations are even more productive at stores outside the airport, often wiping out the dutyfree advantage at Changi.)