Different regions/provinces have different levels of resources that can be mined. The best way to see your projected mining income (if you want to build a mine but don't want to shell out 14,000 mnai for a measly 600 mnai mine) is to place the mine in the construction queue (assuming you have enough money) and then go to the settlement page (you will see faded mining symbols showing how much money you'll get once that mine is built).

I'm also fairly sure that there is a document in EB/Data that could tell you the same info, but I don't know its name exactly.

And any income from mining that is shown is per season.