They pay a flat fee per free game, or at least did: https://www.theverge.com/2021/5/4/22418782/epic-games-store-free-games-cost-apple-trial-arkham-subnautica-m

They pay a flat fee per free game, or at least did: https://www.theverge.com/2021/5/4/22418782/epic-games-store-free-games-cost-apple-trial-arkham-subnautica-mutant-year-zero https://www.theverge.com/2021/5/4/22418782/epic-games-store-...