After much debating amongst a few individuals, I will offer 2.5 credits per gold coin.
I feel that a gold coin is worth more than a single credit, but no where near the value of 10 or 15. That's just ridiculously overpriced. So, 2.5 credits seems fair to me.
Meaning you must sell me at least two gold coins in order for this to work. (because you can't purchase 2.5 credits... )