Pretty much any fee that is predictable. For example, if I would be an oracle and need to post something once every 6 minutes to Obyte DAG and every unit would be 1000 bytes big then I could estimate that I need less than 0.1 GBYTE per year and I would not need to worry how many CryptoKitties games or ICOs are launched during that year, my cost would still be 87600000 bytes per year.