I just did some calculations about the cost of gold coins, and my results are:
Cost of gold mine: 4300 materials (wood, stone and tools)
Ore in 1 gold mine: 300
Ores needed for 1 coin: 8 (2 ores -> 1 bar, 4 bars -> 1 coin)
This means that 1 coin is worth: 4300/300*8 = 114.67 materials
While this in itself might not be a problem (coins could just be that expensive), the problem comes when you see what this means for the upgrading of buildings:
Low level building upgrade 1: 220 materials
Upgrade 2: 450 materials (205% of previous)
Upgrade 3: 500 coins (57335 materials) and 300 materials, for a grand total of 57635 materials or (57635/450*100%) 12808% of previous.
So while the cost of coins itself is not a problem, the cost means that you should never make anything where you mix coins and materials evenly, as is done with all higher level building upgrades.
Obviously you will normally use buffs on your gold buildings, increasing all their output by 100%, this will decrease the cost of a coin to 4300/600*2 = 14.3 materials, but even so the increase in building cost of the upgrade 3 is 1659% of previous upgrade ((500*14.3+300)/450*100%) and this is not counting the cost of the buffs, which is significant when we are talking about such long periods and several buildings.
So I just wanted to say in conclusion that the cost of gold mines should be lowered or the amount of resources in each gold deposit increased, or the cost of building upgrades significantly changed.