Current pay for an f/o with no jet time is HK$53750pm for 50 hours. Avge hours at the moment would be about 70 per month, so that would get you $62750, which is about $53000 after tax.
If you choose to live in Tung Chung or DB, don't expect much (if any) change from $20000, Central or anywhere on HK island will cost you a lot more and you'll get a lot less.
If you're single it's fine, married with no kids maybe okay as long as you can keep the wife happy, but don't even think about coming here on that wage if you have school aged kids.