First some context: I have only been a PC rather than a GM so far, but my brother who is running a campaign (with his own homebrew system based on Shadowrun) with some of his friends asked me for help.
The PCs were granted the honorary title as Lords of an abandoned village as reward for serving as the king’s bodyguard in a battle. The title was only supposed to give them some authority and the possibility to talk to nobles. The PCs however traveled to the village, which had become a bandit camp, and liberated it. They set themselves a goal of rebuilding it and turning it into a small town. They found some former inhabitants and a few beggars who they convinced to join them. Maybe my brother shouldn't have allowed that but by now they already have a trade agreement with the shipyard of a nearby town for a weekly delivery of lumber.
The village is between a river and a forest. Most inhabitants were either farmers or woodcutters before it was abandoned, though it had a smith, hunters, and other professions, too.
I'd imagine there is no need to simulate how the discovery of a new iron ore nearby a town two gaming sessions of travel away would affect steel and weapon prices, but if the players really do found a "trading empire", which I heard two of them discussing on the way out last Saturday, it might become relevant.
My question is: Can you point me to any sources concerning randomly simulated, easily expanded economic system which reacts to PC actions, but also creates challenges for the players to handle? Any suggestions are welcome, if you however point to books as guides I'd prefer if they were available in German, as my brother’s English is quite basic. Not a necessity though. I am aware that the sources might be sparse since I never heard of a pen and paper system which revolves around trading and I have considered suggesting my brother might simply force them out in the world again and killing of the npcs rendering the rebuilding of said village impossible.