i just roleplay it that the "income" mines bring comes from taxs so more slave trade more gold and silver being digged out everything pays taxs and it´s good for the economy it´s like building a road everyone benefits but the state only receives the extra taxs thats increased economical activity creates

so the 5.000 it´s the total that the mine produces but that stays in the society wich keeps people happy wich is more valuable then gold (happy people)