I'd never pay $10,000 for an F ticket NYC to India, but I just redeemed 160k miles per ticket for for 3 of them. The question is, how much did it cost me to get those miles? The answer is: well under the ~$6,000 total cost of 3 tickets on similar flights in Economy - even if you include things like credit card fees for the Chase Platinum Visa (2x miles for "everyday purchases", etc), paying slightly higher prices to fly UA than its competitors, etc. So, $1000 to at most $1500 for 3 F tickets that sell for $30k ain't bad, even if I'd never spend $30k for them.

Now, we could have spent half the miles (80k per ticket) to fly economy instead, and it still would have been a good value. But F is well worth the extra miles - I'd rather take fewer vacations in F than more vacations in Y.

Here's another way to think about value: everyone has a limit as to how much they would spend for an airfare. For instance, I'd have a really hard time convincing my wife to take a trip if the rt fare were more than ~$1500 per person, no matter where in the world the destination was. Every few years, using miles I can get the whole family to some long-haul destination in F for significantly less than that upper limit. How do you value a currency (miles) that can give you something that the cash in your pocket can't, but that you very much want? The value question starts to be meaningless.