Despite the fact that those are much more expensive than silicon transistors, they are so much better that their use saves money in other parts of the system in which they are used.
Diamond transistors could be even better than SiC and GaN transistors, but there are many difficult technological problems that must be solved until being able to make commercial products.
The linked page reports some significant advances, which avoid a source of problems in the past, which were caused by the poor behavior of dopant impurities inside diamond crystals.
What cost? The lab cost today, or the cost after we figure out how to make thousands a day in a diamond wafer fab?
If this were to become practical, operating high speed circuits in space might become a great deal easier due to higher temperature differential. High frequency RF in space is valuable enough that this application is certainly a candidate for such devices, even at costs far greater than Si/SiC/GaN.