It's got to be a lot cheaper. Scrap 2 mirrors, solenoids, penta-prism, focus screen, overlay screen, shims, lots of extra plastic, lots of assemble and fine tuning of moving parts. The sensors have already been developed for video so that's a wash. So it's all that for a oled screen. I got to think that those screens are dirt cheap as they are pretty much in every P?S camera for years.
Differences in EVF quality likely cost a ton more than those physical parts. Chips and processing power is not easy to get down, consider what Nikon and Canon are doing with similar battery capacity to Sony, yet battery life not on the same level. Panasonic has a big battery and can't match Sony either, the R&D efforts go a long way. Not saying it's the EVF for the battery drain, there's a lot of R&D involved to get these high standards.
There's been a lot of tech involved with faster and faster readout speeds, if it were that simple, Canon and Nikon would have produced a Sony killer, that many predicted.

