How is the heroin trail big business?

I learnt about it in a geography class at school.

It starts with the farmer in somewhere, like Afghanistan, he grows it.I then goes to his boss, a local dealer, then a district dealer, then a country Afghan boss, then to a drug smuggler, then to several more of these, then to the boss (like the UK or US) and then to several sellers, to a street seller and finally to a buyer. So you could say it was big business!.