Steam takes a 30% cut, but it's actually apparently worth it for discovery and low-friction sales alone, since developers still sell on Steam when the PC is an open platform with competing storefronts with lower cuts.
Steam also reduces their cut down to 25/20% at $10m/$50m in sales, whereas Apple reduces it to 15% only for devs under $1m/yr. A small number of games/apps make the vast majority of revenue so Steam's average cut is likely considerably lower in practice.
