I don't know if it refers to income per annum but you have to keep in mind that the number of mines varies from province to province. Brettia, where Rhegion lies, has only one mining site and thus a base mining income of 600 (1500 when upgraded).
The richest provinces when it comes to mines are Makedonia and Dalmatia, with 3 mines each, which grants you a base mining income of 1800 (4500 when upgraded).
Any additional income is generated by some trait or ancillary of your governor.
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