Jetstar has an amazing fare sale to Singapore. $379 HKD each way, excludes the taxes. I doubt you’ll ever find a fare cheaper than that.
Total includes the taxes it’s $499 from HKG to SIN, but $591.87 from SIN to HKG due to higher departure taxes in Singapore. $1,090 HKD roundtrip, certainly not bad for a quick weekend getaway.
Book soon as I don’t know when this fare will go away.