The low demand definitely leads to higher prices but the other aspect is all of the additional testing involved for every chip. There's standard testing at every level of the manufacturing process and there will be further environmental testing at the package level. They will even run radiation tests on the wafer to characterize any quirks.
Hopefully as more space & deep space stuff gets going there will be more demand and the unit cost will go down thanks to economies of scale. Or alternatively ways of computing are tried and found viable (massive shielding, standardized massive redundancy, etc.).
