I am still thinking about the buy ticket, get x3 miles then cancel... I would need to find more information on exactly how it is done, what sort of ticket to buy that allows mw to cancel and how to work out what the final price will be in cents/mile...
All of that is very easy to figure out.
What's more complicated and what you need to consider is what the final cpm will be once you've been stung for credit card charges and exchange rate losses.
Let's say as per my other example that you got about 49k miles for $599...for a US credit card, that would be about 1.2cpm.
However, if using an Aus credit card and you had to buy a $6000 flexible J class airfare you might get stung:
Ex rate on the day you bought the ticket 1:1 - let's say you get a rate of 0.98 = AUD6120
Credit Card Fee of 2% = $122
Then you refund the ticket and the rate has changed to AUD1:1.05...the credit card company uses a rate of AUD1:1.07
Your $6000 refund comes through at AUD5600 = $520 lost
Plus they charge you another 2% = $112
So now your USD599 has to have AUD122 + AUD520 + AUD112 added.
Because I can't be bothered doing more sums lets just assume that the rate is 1:1 - so total of AUD1350
Suddenly your cpm = 2.75 - that's much worse than the current promo!
I think this could work really well if you have a USD credit card...otherwise, dangerous!