AnotherGame is a mod for ToE 3.1.

No, no. The market approach cannot be applied in the same way to income. Because a shortage of 1-3 resources is usually discovered on the same day they are needed. The point of the market is that you can exchange any amount of resources at once.
In your case (mines), this is not the case. Therefore, it is more logical to apply the warehouse approach.
As for gold, its payback period is 5 days (like the Capitol branch), since there is no exchange here.

The Capitol is a unique building. A mine cannot violate the principle of a unique building, and the payback period should already be higher. It's not even worth comparing it to a magistrate, because it adds the possibility of an additional upgrade. That is, the payback period for the upgrade should already be higher than 5000, which is why I initially thought about giving 5000, 5 wood, and 5 stone (like for materials for the upgrade). But, although the market is not a complete basis, it does make adjustments, and there can be many markets ;)
Besides, wood and stone mines are captured on the first day, and the creature increase occurs after a week. It would be logical, based on the same price for upgrading all mines (one of the conditions), to set it at 7000. Besides, the player should have common sense that it would be unprofitable for him to upgrade all the mines, and upgrading most of the mines may be possible due to the low price....
The combination of these and some other conditions gives the final price of 7001