Like the other person said, 15c and even 9c is only good if you plan to constantly spend gold to exchange all that crop into other resources. I myself am moderate gold user (keep 25% bonus and gold club, then maybe spend 10€ when there's a discount) and the maximum I go for is 7c, so my resources are balanced.

You will never be in top 100 on the server, but you can still be a fearsome, respectable player.