What cost are you talking about? GameLift is a separate service (if you already use AWS using something like Chef to handle auto deployment is pretty easy and free) and it didn't seem like it was a requirement that it had to be a mult player game so you don't have the AWS cost either.
I agree I don't see what new they offer that will make droves of developers switch. C++ just seems like a bad idea for that kind of tool. At least UE4 has Blueprints that can do pretty much everything you need. I'm sure it will end up being just as popular as Autodesk's Stingray game engine.