Eventually they will be cheaper, but right now there is a lot of cost associated with NRE and reducing power consumption. Reusing parts already designed (i.e. mirrorboxes) is cheaper because EVFs are still improving and there's a lot of NRE associated with that . Once EVFs get retina-like with a high frame rate with low power consumption, then the prices will really start falling.... but what if these parts are more expensive? I do not know the prices of high res OLED displays but if I look at the prices of the EVFs for EVF-less Canon M cameras I do not understand the relatively low price of the M50 ... or the M50 is the first "only-moderately-overpriced" EOS M series camera
Maybe the prices for the OLED micro displays came down within the last year so I would compare the SL2 more to the M50 which are similarly priced (at introduction) while the M50 has an enhanced sensor, up to 10 fps and e.g. the digital balance as benefits.