Pay to post

Hello, great community!

Can anyone help me find where to begin to start or get a function or a plugin that can charge clients (users) to pay before they can publish a post. For example, once a user is logged in, they can purchase any amounts of credits ($1=1credit) through paypal. When they click on 'publish button, there will be asked to choose from a number of pre-defined packages (15, 30, 100, etc), either in a pop up or in a box under the publish button. The post will only be saved to draft for review from me, but only if they have enough credit, otherwise, will be shown a message, saying they don't have enough credit and taken to possible subscription page, within admin.

Thank you