The overage cost is the price per signature on top of the subscribed number of signatures if your subscription reaches its limited. This is billed on a monthly basis.
For example: You are subscribed to a 1000 signatures service with one year duration period. On the 9th month of subscription period, you have used up all 1000 signatures. You can continue to use the service, but will pay $0.90 per used signature. If at the end of the month, if you have used 10 more signatures, you will receive an invoice for $9.00.