Even though your upload quota resets every month, once you upload a podcast episode it will stay there forever until you either delete the episode or cancel your subscription.
Cost for both platforms is determined by how much data (aka how many podcast files, and how big each one is) you upload each month. This amount typically resets at the beginning of each month and can be changed at any time. Even though your upload quota resets every month, once you upload a podcast episode it will stay there forever until you either delete the episode or cancel your subscription.