But node multiplies that, a lot.
Yes. Rails is measured in hundreds per second. Node in thousands per second.
The point that you still seem to be missing is that the monetary amounts involved have normally turned into a rounding error long before you reach a traffic-volume where this difference becomes relevant.
Or, in other words, hosting a "webapp" already is nearly free in terms of hardware.
But node multiplies that, a lot. Which is nice, because you know it won't break or slow down if a bunch of people use it for some reason. And so you don't have to re-architect your system for a while longer, which is valuable time.