Developers that are interested in making games off Unreal Engine will be able to do so for cheap through a new monthly subscription model. For $19/month, you can have access to "everything," including the engine's complete C++ source code.
Once your game is ready to ship, you'll pay a royalty: 5% of the gross revenue of your game. Unreal Engine 4 is available right now.
Do note that "this is the very first release, aimed at early adopters." Current shortcomings include OS X and Android support. In addition, C++ programming documentation "is sparse."
Epic says that "if you're looking for a more polished product, please check back in 6 months."