A common question we get is why manufacturers don't make monochrome versions of some of their cameras. Manufacturing a monochrome sensor would mean leaving out the step where the Color Filter Array (CFA) is photolithography printed onto the surface of the sensor. Yet, monochrome consumer cameras are few other than the expensive Leica M and some really expensive Phase One medium format backs. Why? In a word 'Cost'. To understand why, it helps to understand some things about the semiconductor fabrication process.
I had a customer visit who works for a California company that makes ellipsometers for semiconductor fabrication plants (fabs). Ellipsometers are used to measure the thickness of thin films. Their ellipsometer can measure the thickness of over 150 layers that is common on integrated circuits (IC) wafers. Because the chip layers are so thin, each layer becomes transparent and the ellipsometer can measure the thickness of each layer. Their ellipsometer is designed to spend about 1/2 second at each wafer location before physically moving to another measurement location. When a fab orders and batch of ellipsometers, they send an engineer to the manufacturing plant to interview the entire managrment chain. While the ellipsometers are being built, a fab engineer watches the process at the factory. After the batch of machines is built, each machine is graded to see where in the manufacturing fab line is the optimum position. Even though it is a batch of the same machines, some machines will have higher performance than others. Any machine that goes into a fab being built has to be perfect. The fab quality control is so high, that the fab cannot afford to have a defective machine enter the plant. Once the machine is in the fab plant, the company that made the ellipsometer has to have an engineer at the fab 24/7. The machine is expected to operate 24/7 until the end of the fabs life - typically 4-5 years.
The highest quality control process on the planet (!) is what goes on in a fab plant. Building a fab plant costs billions of dollars. When you are inside a fab plant, the buildings are so big that you can't see to the other side. The fab plants operate 24/7. At the end of their economic life, the entire plant is decommissioned. Every piece of equipment is sold because it can't be trusted in a new fab. Much of the old equipment ends up in university labs, small not-state-of-the-art plants or small businesses like ours. We have equipment that new would have cost us millions of dollars that we bought for pennies on the dollar. Our building is 6,500 square feet and we have lots and lots of equipment. We have (at the moment) 11 spectrometers which can measure from 200 to 1800nm. Spectrometers are built to be optimized for certain spectral ranges so one might be for 200 - 400nm, and another for 900-1800nm, etc. Just in spectrometers, new they would have cost about $70,000. We have an ion milling machine that new would cost around $1,000,000 that we bought for $5,000 (it needed some work but most of the parts where there).
Fab plants current cost typically (2018) 3-5 billion dollars to build. Some plants have costed 10 billion dollars, and the next generation state of the art is estimated at 20 billion. The plants are so expensive, that they sometimes have joint owners and they make parts for lots of different companies. For instance, the Nikon D850 sensor is made in Israel at a fab owned by TowerJazz which is itself partially owned by Panasonic. TowerJazz makes sensors for lots of different companies including Panasonic, Leica, Sony and Nikon. So when you hear about a Nikon sensor, it may be a Nikon designed sensor, but not necessarily a Nikon built sensor. Then there are various iterations where a manufacture may use another companies sensor die (that part of the sensor that has the photodiodes) and then put the die in their own ceramic package and put it on their own PCB built somewhere else. For example, the Fuji GFX-50 and the Hasselblad X1D use the same Sony 50 megapiixel sensor but that the sensor is mounted on their own circuit board.
So, in getting back to our story about why you don't see many consumer monochrome cameras, it helps to understand how these fabs operate. The fabs may be cranking out 100,000 or even millions of color sensors for a particular camera model. For a camera company to ask for 1,000 cameras to be made without out the CFA is cost prohibitive. The fabs are not designed to make 1,000 of anything. Also, keep in mind that it isn't just the monochrome sensor that has to be made, but the camera firmware has to be different, the related camera software, marketing, boxes, instructions, repair parts, etc. It is a big, complicated, expensive undertaking to serve a tiny market. BTW, I have heard from a good source that Leica has lost money on their M because of those very economics.
Fortunately for us, that also results in a somewhat small line of business of converting color sensors to monochrome by removing about 5 microns of the microlenses and CFA from the surface using some of the decommisioned semi fab equipment that has moved down the food chain.