It was a new piece of test/validation equipment used on Hubble first. Reread it, they used the new equipment to test Hubble that had an error during assembly, they also crosschecked against the old machine which showed the mirror was flawed, and incorrectly decided that the newer machine was correct when it passed the Hubble mirror.

The test equipment is a much smaller part of the cost than developing, building and testing the machines required to actually grind the mirrors to spec in the first place.