This would be better as it'd be dependant on how many gold mines you control.
But +20% isn't enough I don't think. How much gold income is each mine? Is each mine just +1? If it is then 4 mines that's not even +1 income. Otherwise it might be okay.
Might want to just +1 income per mine, +2 each mine for lvl2, +3 for lvl3.. But lvl1 cost would need to cost about 50% more then for balance.