From the link: Anyone can ship a commercial product with UE4 by paying 5% of gross revenue resulting from sales to users. If your game makes $1,000,000, then we

From the link: Anyone can ship a commercial product with UE4 by paying 5% of gross revenue resulting from sales to users. If your game makes $1,000,000, then we make $50,000. $20/mo for access to the source code. 5% of revenue to sell something using it.