If you're fitting twice as many transistors into the same space, then the same chip design using the new process will cost about half as much because you can produce twice as many per wafer
You're neglecting the fact that smaller structures need purer "raw" material, which implies much more preprocessing.
Also this calculation only works as long as you can get lamps with sufficiently short wavelengths. If you can't anymore, you have to resort to electron beams, which are slooooow.
To turn a tiny bit more on topic: this a good example that you can't just stop thinking at one abstraction layer, which we recently discussed in another meditation ;-)