I always pick up a room at the hotel desj at changi - which is where excess inventory is sold by hotels. Often the best deals around and often with extras such as free dry cleaining - late check-out, etc. You can call the hotel desk and ask them for some pricing info (phone no. on changi web site) to get a feel for what is available - though you can only book on arrival. I doubt prices would be lower than the priceline offer at marriott mentioned above - but you'd have a large choice of hotels and price ranges.