Nargott
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The problem here is the universality (I want to create something for Might and Magic, and you're suggesting something for the game you play). For Might and Magic, the price is quite reasonable (try playing with such maps; sometimes, there's a catastrophic shortage of one resource), because the maps start with Hugo, and they are rich in resources. But, in principle, the price for each map should be different. It's all relative. Maybe we should switch from gold and create a different price dependency? For example, use other resources or something else?
As I understand it, players should be "so rich they don't know what to do with their money" to afford mine upgrades for 7500. Essentially, my suggestion, specifically for Might and Magic, is to increase the cost of building a castle (including the Capitol branch) or reduce the reward from treasure chambers (and lower the price of mine upgrades).
Right now, the ability to upgrade mines seems like just a way to spend excess resources, because, given even a small choice of where to spend them, the upgrade can't compete with buildings or units.
In my opinion, if a player accumulates excess resources in their strategy (especially gold), it means either they are developing incorrectly, or the balance is off. And this should be addressed not by finding a way to get rid of the excess gold, but by ensuring that there simply isn't any excess in the first place.
Normally, in a situation where I lack a couple of resources at the right moment, I'll exchange them on the market, and gold will be the last resort. Therefore, in my opinion, developing mines is less effective than using the market. In a situation where many resources are needed, you can reluctantly build a warehouse in advance, but often it's easier to buy extra troops with those resources and seize the necessary resource by force.