There is a lot of very risk-averse behavior being suggested here. There is clearly some price at which it makes sense to bank miles. The question is, what is that price? (Whether you ever have the opportunity to buy them at that price is a different question)

I'm going to go out on a limb and throw 1c out there. As long as I didn't have more than, say, half a million, I'd probably pay 1c for them.