AnotherGame is a mod for ToE 3.1.

Perhaps there is only one such branch for a single player game. What difference does it make? The essence won't change.

The difference is significant. What will I build: a municipality for 5000, which gives +1000 to gold, or an upgrade to a gold mine for 7500?

You would build a warehouse much later than you capture the necessary mine anyway.
And how will you build a wood warehouse?

Compared to wood, yes. But not compared to a specialized, rare resource.

Added 2 minutes later

The Castle takes 20 days to build. That's an increase of 40 stacks of wood.
Is it worth paying such a price for 7500?

I don't understand the point; what does the Castle's construction time have to do with the mine's output? I will only get 40 stacks of wood by the end of the period. Be consistent with the payback period – 7 days means 7 days, not 20.
In the case of the Elves (and I would never trade 7500 gold for 40 wood for other races), a lot of wood is only needed in the first week.