The cost of credits is: $10.00 - 10 credits, $20.00 - 22, $30.00 - 33, $40.00 - 45, and $100.00 - 110.

So the cheapest per-credit cost is at $40. And there is no price difference between the $20, $30, and $100 options as they all give 10% more credits than dollars.