Lower cost of Endless Copper Mines. They are nowhere near worth the value.
Anyone now can sell in game at a 1gem=3gc conversion rate Minimum... Most is converted to a 4 or 5 gc conversion rate by selling nobles or golden parrots.
so 20k gems= a minimum of 60,000 gold and maximum of 100k gold using basic math.
I will spare you the math on the amount of copper you would need to sell at the going rate to come anywhere near those numbers and how long it would take you even buffing the copper mine at level 5 24 hours a day.
Even with a 100% xmas sale it is just silly to purchase an endless copper mine. Also I can buy 3 recycling manufactures and take advantage of the free one provided and the xmas event one which is roughly 4k gems... and the production would be nearly equal in gem to gold production.
This is something the devs should think about and take into consideration to change soon.