AnotherGame is a mod for ToE 3.1.

The high cost of selling resources on the market is intended to prevent resources from becoming equivalent to each other ;)
That's why I suggested shifting the upgrade to other resources, so it wouldn't affect the amount of meat available for purchase. Although, for a breakthrough, only 3-4 types of creatures are sometimes bought, which is enough.

Because your rate (7500) is even less profitable than what the market and warehouse offer. The point is to make it equal to the market, i.e., roughly speaking, to make mine development analogous to a long-term market (a combination of exchange and income).

In general, to be honest, I'm starting to seriously doubt the feasibility of upgrading mines. As a rule, only a rare, standard resource is in acute shortage, and this problem is solved by the warehouse. The exception is perhaps only the elves (wood). A non-acute shortage can be solved by the market, practically without any gold investment. Perhaps only upgrading the gold mine can play an important role, and even then only as a supplement to the municipal branch (i.e., due to the price, it's not the primary factor).

Added 3 minutes later

That's why I suggested shifting the upgrade to other resources, so it wouldn't affect the amount of meat available for purchase. Although, for a breakthrough, only 3-4 types of creatures are sometimes bought, which is enough.

It doesn't solve the problem of resource efficiency, but it does allow you to reduce the burden on gold and increase the use of surplus resources. Therefore, I support this. The other question is whether upgrading a mine for a specific resource will be worthwhile.