Ensure registered users can publish nodes only after paying some money
The pre-existing solutions ( Commerce Node Checkout,   UC Node Checkout, Pay-per-node) for this problem haven't been ported into D8. 

If one of the pre-existing solutions covers your use case well, you could help port it to D8 (Commerce 2.x is likely to have a stable release this year). Otherwise, you're looking at some sort of custom solution involving involving modifying the node form workflow and PayPal integration.