Check forex / international transaction fee. This is the amount you are purchasing your points for. The value of your point will vary depending on how you use it.
If the fee is 3%, and your card earns you 2 points per dollar, each dollar you charge to the card results in the following
1. 2 points credited to your account
2. 3 cents charged to your account
This means you have bought a point for 1.5 cents each
Now if you then use your points to pocket yourself return business class flights to London, you'll be up for 256,000 Qantas points and about $900 in taxes and charges. This would usually cost $7,800 but because you pay for fuel surcharges and taxes on top of the 256,000 points, this means the points are only worth $7,800 - $900 = $6,900.
What is your point worth in this redemption?
$6,900/256,000points = 2.7c per point which isn't bad considering you paid 1.5c per point as per the first calculation.