If I could buy miles, one cent is my threshold. For United M+, that adds up to $250 for a domestic Coach ticket, $1,000 for a First Class ticket to Europe. Since I'd never pay for International First out of my own pocket, miles are the only way I get to enjoy this luxury.

One cent is a useful benchmark to figure out how much my various mileage schemes are really worth. For instance, paying $60 a year for a credit card that yields 25,000 miles per year is a good deal. Paying $350 for a DEN-ORD ticket that yields 4,552 miles (air miles + 1P bonus + booking bonus) is not.