Ahh, ye olde mines were great 15,000 mnai income from a single settlement (Pella ) was a bit excessive, though...

To answer your question, it should be in export_descr_buildings.txt. Just do a search for "mines." Here's the 0.73 entry:

Code:
building hinterland_mines
{
    levels mines mines+1
    {
        mines requires factions { barbarian, armenia, romans_brutii, egypt, romans_scipii, carthage, parthia, numidia, thrace, greek_cities, macedon, romans_julii, seleucid, } and resource silver or resource gold
        {
            capability
            {
                mine_resource 60
            }
            construction  4
            cost  10000
            settlement_min town
            upgrades
            {
                mines+1
            }
        }
        mines+1 requires factions { romans_brutii, egypt, romans_scipii, carthage, parthia, numidia, thrace, greek_cities, macedon, romans_julii, seleucid, } and resource silver or resource gold
        {
            capability
            {
                mine_resource 200
            }
            construction  10
            cost  20000
            settlement_min large_town
            upgrades
            {
            }
        }
    }
    plugins
    {
    }
}
Hope that helps.