This is why I try and keep my iron mines up to highest lvls and my lumber mills as well. They sell for the most money. Iron seems to build faster for me even at lvls below 8 I can get 1mil in iron in a day or 2 with only 6 mines. Attacking and keeping high lvl hills for each city helps as well.
Remember you need to take valley resources for each city you have. The % gain for taking a valley is only for the city that sent the troops.
Happy hunting!!
