Bob May writes:

You can always make 3 flats for a very low cost.

Yes, but my impression is that the 3-piece process for originating flats is
simple but extremely laborious. Besides, I already have a surface plate
and would use that to scrape a steel straightedge if they're going to cost
$100s to buy. But if a granite straightedge could instead be simply cut
from a $30 granite surface plate, it wouldn't be worth the effort to
originate any of the surfaces.