Sydney has pretty decent public transport, at least by Australian standards, and most certainly compared to any US city of comparable size. I never park my car in central Sydney on a weekday, and have never been particularly inconvenienced by that cost.
Yes after 5pm weekdays and on the weekend there are plenty of flat rate places for $15 or about $10 more towards the north end of town.
I just wish they'd build more parking near some of the suburban train stations. It's usually OK far from the CBD, but a park and ride strategy would definitely help at places 15-20 minutes out. Mascot and Green Square have both gone through a lot of development, are expecting a lot of new people, and still have tiny roads with no dedicated parking near stations.
The CBD shouldn't be for cars.