As Pete R said $50 - $55 would be about right for the taxi fare, if you go through via O'Riorden Street through Redfern and then on Abercrombie Street to the Anzac bridge at the Fish Markets. This avoids the tolls and is the way I always go. At that time of day it should be about the same time. Saturdays in Sydney can be a lottery with one way been as bad as another for traffic.
It should take you about 30 minutes but I would allow 40. I can do Gladesville, which is the next suburb out on Victoria Road across the river, to the airport in just over 20 minutes, but only when heading for the 6am flight, which is a very regular occurrence at the moment and is what I have to look forward to on Monday!
Cheapest way is train and bus. If everything went like clockwork this could only be about 45 minutes but you would need to allow at least an hour taking into account waiting for connections etc. The train is $15 single, but I think they do a day return which is a little less than two singles at $15 each.
If you are really in a hurry and don't want to run the risk of having to wait for a cab at the airport try www.towncars.com who do a fixed price quote for that trip of $56 including everything except credit card surcharge and will be waiting for you when you walk out of the terminal.
