According to the passage, what is a "margin"?

Every evening, Kamla sold vegetables from a cart at the corner of Gandhi Road. She had never been to school, but she could add up a bill for six kinds of vegetables, take a five-hundred-rupee note and count out the change faster than any customer could check it on a phone.

One evening a college boy bought two kilos of onions, one kilo of tomatoes and half a kilo of beans. Kamla said, "One hundred and forty," before he had even put his wallet away.

The boy took out his phone and did the sum. "How did you do that so fast?" he asked.

Kamla laughed. "Onions are forty, so two kilos are eighty. Tomatoes are forty. Beans are forty a kilo, so half is twenty. Eighty and forty and twenty." She shrugged. "I have been doing this since I was ten."

The boy studied commerce. He told her that in his class they were learning about profit and margins, and that he found it difficult. Kamla asked what a margin was. He explained: the difference between what you pay for something and what you sell it for.

She thought for a moment. "I buy tomatoes at the mandi for twenty-eight and sell them for forty," she said. "But one kilo in every ten goes bad. So my real margin is less."

The boy stared at her. "That," he said, "is exactly what our professor took an hour to explain."