How much do I have to pay for Unreal Engine 4?
UE4 is free to use, with a 5% royalty on gross product revenue after the first $3,000 per game per calendar quarter from commercial products. Read the EULA FAQ for more details.
I’m a consultant. Do I owe royalties on consulting fees?
I think the reason for this is they all want to become the defacto-standard, they are all very keen to create a developer community around their toolset. Personally I like the UE4 / PhysX sales model since you don't pay until you make money from it. I'm interested in playing with these engines as a hobby but have no interest in writing a commercial game, If I was serious about developing and selling games, the license fees for any of the popular engines would be a very minor concern, it's a great example of a capitalist "win-win".
Selling model content to use in these engines is where the money is for individual devs/artists, kind of like the people who sold shovels during the gold rush. IIRC UE4 has some sort of public marketplace where you can release/sell models you have created.