They didn't have a monetary system as we know it. Members of their society had to dedicate a proportion of their labor to the empire, to the emperor, and this might seem different than what we do, but if you think about it, if my income is taxed at 30% or 35% in, say, the US, essentially what I'm giving to the government is 35% of the work of my labor. I'm just doing it through a monetary system. They did it directly. A certain percentage of my labor would directly have to be for the empire, and that's how things like Machu Picchu actually got built. This was called the Mit'ah system. Well, like the case with the Aztec Empire, the Inca Empire lasts for about 100 years until the conquistadors come into the picture, and Francisco Pizarro in particular on his third expedition is able to conquer this powerful Inca Empire, and he does this with only several hundred men, but they were far better armed than the Incas, but many historians believe it was also complacency on the part of the Inca ruler at the time.