For sure prices in downtown Singapore are better than Changi. In any event, if you have to close the deal at the airport, I will again stress that you must negotiate in two phases. First, haggle with the salesperson until he doesnt offer you any more discount. Then, make sure you convince him that you will buy the item if he/she goes the extra mile and ask him to phone a supervisor and get the approval. This will usually yield the best possible price.