It's not by much, but the cheapest is now the bus to Mascot (350/420 from domestic or 420 from int'l) ($3.20 in peak) and then the train from Mascot to the city ($4.20 in peak). With the $2 interchange discount, that's a total cost of $5.40.Sydney Airport (to Circular Quay)
Fastest - Direct Train - 27 minutes -$16.77(now $21.54)
Cheapest - 2x Busses - ~1hr 11minutes -$4.71(now $5.60)
Off-peak works out to be $3.18.
It does involve a 3-4 min walk at Mascot though.