AnotherGame is a mod for ToE 3.1.

Simple math. The production cost of a rare resource is 200, and it can be exchanged on the market for 5000 gold. That's a difference of 25 times. But you can exchange any resource for any other. In the case of mines, you can't exchange just any resource for any other; it's mostly gold for any other resource. It would then be logical to have a difference of 5 times (the square root of 25) for the halved restriction on freedom. But then we also get an equivalent income of 1000 per day, and a payback period of 5 days = 5000.