The cheapest way to get from Sydney Airport to the CBD

If you are arriving at Domestic Terminal 2 or 3, take the 400 bus (outside Terminal 3) to Mascot Station (the next but cheaper underground station) $2.30 Adult fare. It's the first stop after the bus turns right off Bourke Street. You can buy your ticket from the driver but have the correct money. Or PrePay, buy tickets at shops displaying PrePay signs.
The 400 bus stops at International Terminal (T1) before T3.
At Mascot Station the fare is $3.80 (as of February 2014) to St James (Hyde Park) or Circular Quay (Sydney Harbour).
If you take the train from Domestic underground station (under the terminal) it will cost $16.40 Adult one way.
International Terminal (Airport Link) costs $17.20 for same destination (but if you are jetlagged or you are travelling with 2+ people), I'd taxi it or take that train from International underground station (if travelling solo).
In summary: $15.90 train or $6.10 bus+train.
The same 400 bus continues onto Bondi Junction from Terminal 3. Bargain.