I think it depends on which terminal your transit flight will be, however for Ambassador transit hotel, there are available for all terminals. Single budget room starts from 42 SGD (minimum charge of 1 block of 6 hours)
For Reservation Bookings Reservation Office open from 0900hrs to 1900hrs (Singapore Time) E-mail: mailto:rsvn@airport-hotel.com.sg More info:
http://www.airport-hotel.com.sg
Terminal 1 Departure Transit Lounge East, Level 3
(Go up the escalator near Gate D40)
Tel: (65) 6542 5538
Fax: (65) 6542 5537
Terminal 2 Departure Transit Lounge South, Level 3
(Go up the escalator near Koi Pond)
Tel: (65) 6542 8122
Fax: (65) 6542 6122
Terminal 3 Departure Transit Lounge North, Mezzanine Level (Level 3)
(Adjacent to Ambassador Transit Lounge and Movie Theatre)
Tel: (65) 6507 9788
Fax: (65) 6242 8542
Advance reservation of 3 to 4 weeks is advisable.