If you're looking for a reasonable, all inclusive rate, go to http://www.britz.com.au
Their main business is renting campervans, but they also rent regular cars at very good rates which include all fees and insurance with excess reduced to $220. Cars are sourced through Thrifty 9one of the major rental companies here), so they're all new, reliable vehicles. You can pick up from the Thrifty counters at the airport or from the main Thrifty office on Wiliam St which isn't too far to walk from Darling Harbour.
I've used Britz a dozen times or more, and haven't had any problems apart from very occasional inexperienced Thrifty counter staff who aren't sure how to put the Britz rate in their computer system (They are often surprised how cheap the rate is). There's always been another member of staff on hand to sort it out quickly though.
The only real drawback of using Brtiz is that you have to pay 100% up front, so if you're travel plans are delayed or changed, you *might* end up being out of pocket if you miss their cancellation deadlines.