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.
Off-peak works out to be $3.18.
It does involve a 3-4 min walk at Mascot though.
