When deciding whether to use miles or cash, I take into account net loss of miles, that is by using the 60,000 mileage award for PIT-PVG, you're giving up approx 15,000 miles you would've earned by paying the $900 fare. Thus, $900/75000= 1.2 cents/mile. In my own case, it would've been even lower at 1 c/m as I get a double point bonus for being AA platinum. I generally like to get at at least 1.5 c/m for those hard earned miles. cheers.