When comparing the price of technologies, it's important to consider the real costs, not only what seems to be more expensive. Although a mirror/viewfinder is certainly more mechanically complex than an LCD and a few cables, the SLR technology is very mature, and there's little innovation needed. Most or all of the parts can be manufactured without human intervention and assembled by low or unskilled labour, often in low cost countries like Nikon does in Thailand. The big difference compared to electronics is that the price won't go much down with increasing production figures because of the manual labour part, but since mirrorless cameras are produced in relatively small numbers, both as a total and not least because there are many suppliers and even more models per supplier involved, they don't really harvest the electronics advantage. DSLR manufacturers have the great advantage of being only two and a half (that would be Canon, Nikon and Pntx), with camera models that are manufactured without much change year after year.
For some reason, this reminds me of a conversation I had with an engineer at Xerox when I worked there back in the day. I was impressed with the speed of the top model, which at the time was the Xerox 9500 that could copy 120 A4 sheets per minute, so I asked this slightly naive question: how is that even possible.
The answer was still interesting: You design and optimise every little part in the mechanical paper path using very small tolerances until there's little resistance, neither mechanically nor aerodynamically. The next and obvious question from me was then: Why aerodynamically? The answer to that is that the next challenge, and the one that ultimately decides the maximum number of copies that can be produced per minute, is the aerodynamics of a single, 80g sheet of A4 paper. So the limit for single sheet paper is absolute, and the only way to get around it is using paper on roll, which is what IBM did with the 3800 laser printer.
Here's also one clear parallell to DSLR vs. mirrorless cameras. There's a physical limit as to how fast it's realistic to move the mirror and a mechanical shutter (unless one uses rotating shutters like in cine cameras). By using electronics, there are no such limits, at least none that we need to worry about. The question then is how useful these high shutter speeds and frame rates are for "ordinary" people, which is probably one of the reasons why DSLR cameras still survive and sell in higher numbers than the mirrorless alternatives.