R to @fchollet: Marginal cost is not a detail. It is the whole thing. Imagine going to an airline, and they tell you, "yes, we can get you to 75% of these top destinations" -- great, but how much does it cost for each one? And how long to get there?

<p>Marginal cost is not a detail. It is the whole thing. Imagine going to an airline, and they tell you, "yes, we can get you to 75% of these top destinations" -- great, but how much does it cost for each one? And how long to get there?</p>