I don't know if this is a brainstorm or a drizzle.
Upgrades are 31.50/ each when you buy more than 8 --- so 10 = 315.00
if I can turn in 4 for 10,000 miles than 8 for 20,000 would come to a cost of a little over .015 cents per purchased mile.

Is this allowed?

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TANSTAAFL - but if you work it right, FF miles comes pretty close.