Most people reckon extortion is the biggest source of income for these gangs. There's big money to be made in sectors like agriculture and mining. Mexico exports $3 billion worth of avocados every year, mainly from western state of Michoacan. One man said for the last three years he's paid an annual fee of 10,000 pesos (about $560 per hectare) The gangs come with data on the size of his farm. They tell him when to hold back stock to push up prices.