Unity, Unreal Engine 4, and Source 2 are free for game developers and modders to use, and each platform has its own way of monetizing.
Epic Games announced just a couple days back that the Unreal Engine 4 would be free, with royalties on successful releases.
Later, Unity 5 was also revealed to be relatively free–with a subscription fee or one-time payment–with its CEO John Riccitiello (formerly of EA Games) denouncing royalty schemes as "fucking around."
Not one to be left out of the engine race, Valve has quietly announced Source 2. It's also free, but with one caveat–games developed on Source 2 must be released on Steam.
Valve notes that games developed in Source 2 will not be prohibited from being released on other channels, but it has to have a Steam version. It only seems fair, as Valve gets a cut out of every sale of games sold on Steam, through Steam. Valve's Erik Johnson disclosed the requirement to Rock Paper Shotgun.
The only downside to using Source 2 exists if you don't care for Steam's various features and do not wish to release the game on their platform.
