Tiger airways has opened another flight for the Perth to Singapore route. I've managed to book a flight from Perth to Singapore Return for $420 incl tax.
Not sure of the exact dates when it has cheap airfare, but try booking dates (1st or 2nd week of July) departing Perth to Singapore and (2nd Week of July or 3rd week of July) returning.
Add $5 to the cost each way if you want to pick your seat and add $25 if you want that seat to be in an exit row. Exit row seats get a massive amount of legroom - I was quite comfortable on my recent flight to Singapore with Tiger.