I've been out of touch for a while -- what's the driving factor in mask expense these days? Time in the mask writer? Something about only being able to pack so many simultaneous electron beams next to each other before they fuzz out?
To push a technology like DUV (deep ultra violet) further and further requires multi-patterning, which means several masks for a single processing step.
For EUV (extreme ultra violet) no material is transparent so the masks are actually mirrors. With 40 alternating silicon and molybdenum layers, each the exact thickness to match the 13.5nm wavelength, you can reflect enough of the EUV beam to get the job done. Making such a mask is a lot more expensive than one that is basically a glass photographic plate.
The time to actually write a mask has gone up by a large factor, as you mentioned.
The combination of all the factors increases the cost of the masks from a few thousand dollars for 1970s technology to many millions today.
Lots of chips in use today are still not anywhere near the tiny end of the scale, and they're clearly still being manufactured somewhere, but it's like economies of scale have somehow worked against price in IC manufacturing in some ways (but obviously not others: per-feature-count is obviously much more cost effective now).