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The spherical aberration of Hubble's main mirror was caused by a manufacturing defect.

Perkin-Elmer designed a new null corrector to verify that the mirror was made to spec, but the null corrector was incorrectly assembled (one of the lenses was out by a few mm). They also used the previous null corrector to test, which showed that the mirror was incorrectly ground, but decided that the new null corrector's measurements was correct.

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But wouldn't they have needed to do that for KH satellites too? If they're making new equipment and processes, they'll be saving a lot less money from using KH mirrors

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.

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