The mongols developed a nomadic society in their central asian homeland with an economy based on

The best answer would probably be trade when considering the duration of their whole empire. 

However, a good portion of the starting capital Mongols had was received by looting and ransacking cities. The Mongols were actually notorious for ransacking cities and taking everything that was valuable, also killing the local population. 

They did, on the other hand, also establish trade with different neighbouring nations which allowed them to gain additional streams of revenue.