The large die is due to still being on 12nm. 7nm was just not ready and even although die sizes would be smaller the costs would end up far higher.
Some recent IR shots of the Turing shows that RTX and Tensor cores are about 8% of the die area, so they have very little no added cost.
Given 12nm is more or less just the same as 16nm, for Turing to be a successor to pascal required the die sizes to go up a lot. Unlike pascal there was no real compute stuff to take out and Pascal was already very lean.
I think you are also way overestimating the die size costs. Even with the extra 8% for RTX & Tensor cores, the Turing chips would be cheaper than Pascal to produce for a given number of compute units etc because yields are higher on the more mature process.
The price difference doesn't come form the dies, but the increasing R&D costs, increased memory costs, and increased margins.
If the RTX & Tensor cores were not added total manufacturing costs would be a few percent less at most, basically indistinguishable. Having 8% more CUDA cores might seem nice but in all likelihood is just not possible because core counts always have to scale in a particular way (normally following a power of 2).
When RTX is impacting prices it is purely due to marketing. Nvidia believe they can charge more due to the features. The market will decide and if they disagree with Nvidia then NV will simply reduce margins. Nvidia's statement to investors is so far Turing has beaten Pascal in revenue, that is all shareholders typically care about. Also, Turing was the fastest growing GPUs on steam survey, and nvidia;s market share is only increasing. So most of the siogns point towards Turing selling quite well in and of itself.