Originally Posted by MiracleSurvivor
Trade (or Capitalism if you want to get fancy) is based on supply and demand. While fewer people means fewer buyers/consumers (thus lower prices), fewer people also means fewer seller/producers (less competition). So in the end it doesn't have any effect at all.
There are two exceptions though:
- Mass production, the more products are produced the cheaper every unit gets. This is mainly because of the massive upfront cost of production (think the factory where cars are made).
- Land price, since land is finite and can't be created. It could mean that land becomes cheaper as fewer people exist on the planet.