So, the promise of Evony is "always free to play". This is categorically true. So, where does the money come in? Time. The producers of Evony are banking (quite literally) on people becoming impatient with the pace of waiting for resources (food, lumber, stone, iron and gold) to accumulate, or for construction or research to complete. Their solution? Buy game coins. Pay real money to get game money with which you can buy items (such as speedups or production buffs) or get more resources outright.
I can only assume that these folks are raking it in. They even have a feature called "Auto Cents" where they will automatically charge your credit card... Indications inside the game are that players spending $100+ per month is not unusual. Wow.
Me? I'm taking the slow road.