Just adding processing steps requires more masks. Moving from visible light to smaller wavelengths requires more and more sophisticated materials for the masks

Just adding processing steps requires more masks. Moving from visible light to smaller wavelengths requires more and more sophisticated materials for the masks with smaller and smaller imperfections. 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.