In our privacy policy page we did mention that third parties may have access to your information.
is this even possible? how much we should charge per email blast?
It's 100% possible since the emails are in your hand. If the policy was third parties may have access of the information then you are okay to go. However in normal case a subscriber is interested to the product or service you offered and gave you their details for that reason. They did not gave the details to receive spam in their inbox.