Analee Newitz in io9:
One of the outstanding questions for scientists and historians who study the Incas is why this wealthy, sophisticated culture developed scientifically and culturally without ever inventing markets. One possibility is that life was so difficult to sustain in their environment that all their innovations revolved around agriculture rather than economics. In other words, the Inca Empire was optimized to prevent starvation rather than to foster trade.
A few years ago, a group of archeologists took core samples in Cuzco valley in Peru, and found evidence for thousands of years of agriculture in the area, including animal husbandry, most likely of llamas. In a paper summarizing their findings, archaeologist A.J. Chepstow-Lusty and his team suggested that the Incas focused their technological and cultural institutions around food production and land management, rather than market economies. This may have been necessary in a region where droughts had likely wiped out a previous civilization (the Wari), and where climate fluctuations were a constant hazard. The rise of the Inca Empire coincided with a period of relative climate stability, but the peoples in the area would be well aware that this temperate spell could end at any time…
So how do you become a continent-dominating empire without cash? In the case of the Incas, it's likely that the technologies that granted them agricultural surplus (extra food and textile materials) helped them with their expansive empire-building. Food was their coin; pure labor structured their economy.