Developers set their own prices through strictEq.
Developers pay Artless Devices, the company behind strictEq, commission on each sale. Commission is currently 5% of purchase price for new accounts. See the commission section of the agency 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. strictEq can’t do your taxes for you, but it reports buyer and seller jurisdictions, so everyone has records.