AnotherGame is a mod for ToE 3.1.

The Town Hall - Capitol branch is unique in itself, just like the Capitol itself, because you can't build another branch like that, nor another Capitol ;)
And what you're saying is something else entirely.

How can you not build it - another town? Forget about the Capitol and its uniqueness, we're talking about the Town Hall now.

I'll repeat myself, but try to put it in other words:
Upgrading mines shouldn't become the norm.

And it won't, with prices at 5000. Because, as a rule, I rarely even build a warehouse.

Have you even played on IRMG maps? For small maps, 5000 will be optimal, and even 10,000 will be too little for large maps.

If you're balancing for a long game with a payback period of 7 days, then you should also increase the prices for the Capitol branch and the warehouse. Right now, the Town Hall and even the warehouse are primary, while mines are secondary.

Don't forget about conversion, which gives you a chance to build a second Castle ;)
The average will definitely not be around 5000, and this is also a plus to what I said.

I don't know how you play, but I always build a second Castle, using the surplus after the first one, without any extra conversions.

Added 1 minute ago

For small maps, 5000 will be optimal.

What do you have in G5 on IRMG for small maps, because, for example, in G3, I don't even bother with the Town Hall and the City Hall on small maps, due to their unprofitability.

Added 31 minutes ago

Calculate how much will be missing with a normal Castle build, without comparing it to any race. That will be more accurate. Humans also sometimes lack stone, and less often wood.

Why without comparison? For humans, in the vast majority of cases, I haven't encountered a shortage; and the cases where it was, are offset by cases where there was an excess of wood. Therefore, for elves, in most cases, +12 to wood will be enough, the rest is surplus for exchange.

However, I have presented all the arguments, so I'm ending the discussion. It's up to you to leave it at 7500 or not. In my opinion, such prices are an oddity, even more so than a warehouse.

Added 4 minutes ago
Another question, how are mines upgraded? Does it require a visiting hero, or is it a global spell?