Learn Spanish (Latin America): Aunque es caro, el margen es el doble. - Although it is expensive, the margin is double.

In Spanish, multiplicative terms like doble (double) or triple (triple) act as nouns when they stand alone to represent a quantity. Because it functions as a noun meaning "the double amount", it needs the masculine article el. In English, we just say "is double", but in Spanish, you literally say "is the double".

The lesson mentions using el doble de. Why isn't there a de in this sentence?

You only use de when you are explicitly stating what it is double of right after the phrase. For example, you would say el doble de grande (twice as big) or el doble del precio (double the price). Since this sentence just says "is double" and ends there, leaving the comparison implied by the context, you drop the de.

Why is it Aunque es caro with the indicative, rather than Aunque sea caro with the subjunctive?

Using the indicative (es) here means you are stating a known, accepted fact: "Even though it IS expensive..." If you used the subjunctive (sea), it would change the meaning to a hypothetical: "Even IF it were expensive..."

Is margen masculine just because it ends in -en?