AnotherGame is a mod for ToE 3.1.

Simple math. The cost of a rare resource is 200, and it can be exchanged on the market for 5000 gold. That's a 25-fold difference. 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. It would then make sense to have a 5-fold difference (the square root of 25) for a halving of the freedom of choice. But then we also get an equivalent income of 1000 per day, and a payback period of 5 days = 5000.

Let's analyze it using the Elf example. 5000 is the maximum price that I personally am willing to pay for a shortage of wood. The same applies to mercury for the Undead. The Warehouse is slightly more expensive (+5 stone), but its advantage is that it can be built at any time, including at the beginning of the game, and not only when the mercury mine has already been captured (and development only speeds up the growth).