I was going to say something about contracts, where an agent would gain access to a service but with some commitments. e.g. after buying a monthly bus ticket, the agent has a 'sunken cost' that it has to consider when evaluating other forms of transport. So if a better option pops up, they likely won't change at least until their monthly ticket expires.What would you like to see in an agent-based economics simulator?
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Farms complicate things. Is a farm one 'building' or is it made of grain silos, beehives, etc.?
You could use these kinds of contracts to build hierarchies to solve the farm issue too. E.g. a field can be worked to produce food, but it's owner has specified that that workers can only deliver the food to a specific silo (also belonging to the owner).
As with the bus ticket example, the farm owner might sign a distribution deal with a particular retailer for some time period, where breaking the contract means he has to pay a penalty fee. The retailer may sign a contract with a particular trucking group to move food from the silos to the retailer, etc, etc...
If you could build this reusable economics engine as a stand-alone library, which games could use to create their economies, then I can picture it being quite popular among indies that want to make these kinds of RTS/city-builder games, but don't have the time to build the simulation engine.