My hubby has taken our son to Sydney Airport to watch planes before. Not as expensive a trip for us (already in Sydney, and son was under 3), and they both enjoyed it. I don't know if it's worth $60 though. If you don't mind walking, you can get off the train at Mascot station and it's an easy 10 minute walk - saves a fair bit of money!
Our son loves Luna Park - he and hubby went yesterday.
ETA: if you go on a Sunday it's $2.50 unlimited transport BUT you still have to pay a station access fee at the airport stations - info here
This post has been edited by Jekaho: 02/01/2013, 12:42 PM