Developers set their own prices through Indie Code Catalog.
Developers pay Artless Devices, the company behind Indie Code Catalog, commission on each sale. Commission is currently 5% of purchase price for new accounts. See the commission section of the seller terms for specifics.
Developers also pay for Stripe payment processing. As of May 2020, Stripe charges 2.9% plus 30¢.
Depending on the jurisdictions of the buyer and the seller, one or both may owe sales taxes. Indie Code Catalog can’t do your taxes for you, but it reports buyer and seller jurisdictions, so everyone has records.