Members versus Listings

Currently on my website, I have four membership levels. Each level can have up to a certain amount of listings. I set it up this way when I built my first premiumpress site. Below is my goal. Is this doable? If so, how?

Ideally, I would like one user to be able to buy an infinite amount of listings, but they must pay for each individual listing.

For example, if a user wants one listing, they can buy it for $10 a month. If they want two listings, it costs $20 a month. Is the site designed to do this?

Thanks for your help. Sorry if I missed this somewhere.