1. Check air ticket price on online travel agents websites such as zuji.com.sg or priceline.com.sg
2. Check for airline that offers the cheapest price and go to the airline's website to buy the ticket.
This should save you at least 30 to 40 dollars as online travel agent fee.
For budget airline, I realise that no matter how cheap other air lines are Jetstar will be cheapest. Just find any ticket cheaper than Jetstar and Jetstar will beat the price by 10%. More details can be found here. http://ssl.jetstar.com/sg/en/travel-info/price-beat-guarantee.aspx
