I can't build a decent car for less than the cost of a low end BMW. But, there are plenty of other companies than can.
So, the real question is what are the other options out there and I suspect there are few engines that can come close.
However, UE4 is much more than just a graphics library. I'd argue that it's editor alone is worth many times more than their rendering features. And IMO, it's trivially worth the 5% of revenue.