On top of base prices, allow players to configure purchase prices for materials. The money will get stashed in the actual area until it gets spent, withdrawn or the area is cancelled.

Other players can then sell materials to the area for the configured buying price. This can happen even before the plan is finished and surplus materials will be available to the owner in the supply chest

Because this will be a fairly complex set of operations, implement this in spout first and then as a set of commands for clients without spout