Hi,
I can't see that. It's the same sensor. It goes in the exact same way. So, no mfg cost differential there. The sensor itself probably costs a little more unless the maker decides to add the mono in with the color for purchase quantities. There are always component price breaks based on purchase quantities.
The real difference is Fuji would have to add code into their processing firmware to deal with not having to process the color filter array. And that's probably the Why of those not doing it. You can modify as you wish and the use an alternative post processor on then the Raw file.
Stan