Terraforming Mars $70.37 + Int. Delivery (Free with Prime) @ Amazon AU
I've been keeping an eye on this for a while, not the cheapest of all time but a great price for (what I've been led to believe is lol) a great game. Best local price is $84 + $10 shipping at Gamerholic.
Seeing as I tend to see a lot of comments asking about suitability for children and is this a good place to start, etc I'll just say that this is a fairly significant step up in terms of complexity compared to "gateway game" staples like Catan, Carcasonne or Azul. If you're interested in engine building (which is such a fun mechanic) but worried this might be a bit too full-on I would recommend looking at the following games first: Lightweight - Splendor, Machi Koro; Middleweight - Wingspan, Gizmos, Race for the Galaxy, 7 Wonders/Duel.
Even if you know you'll love this they would still be good options because TM is going to be harder to get to the table… but I'm going to try lol.