If you want a cheap flight from Perth to Singapore, you've got zero airlines to choose from.
Singapore Changi International Airport is just one of the airports in Singapore. If you're not set on a specific airport, try changing your search from Singapore Changi International Airport to Singapore to see a broader range of flights – they might be cheaper.
If you're planning on going to Singapore during this time, you could take a look at other airport options. Or you could try to get a last-minute bargain.