Hubble's cost structure is complicated. It is essentially a modified spy satellite and so much of its development costs were shrouded. We don't know how much was saved, or how much of Hubble's public budget was used on other projects. JWST is different. It is a unique object with no equivalent. Its cost structure is therefore not comparable to Hubble.

(Or I am wrong. Maybe JWST is a modified version of a super-secret unannounced program and its budget is covering the development costs of that spying program. Given the testing scheduled I doubt this, but it remains a possibility.)

Part of the cost of Hubble was that the teams working on the keyhole satellites was not allowed to tell the Hubble designers about "lessons learned" and the Hubble team had to find them out the hard way.

Any links or references? Sounds like a fascinating backstory.

I think it's fair to say that JWST isn't just a tweaked KH like Hubble. The mirrors and sensors in JWST are completely optimized for infrared, which is a borderline-useless part of the spectrum for high-res terrestrial surveillance. Optical scopes also wouldn't need the extensive sunshield and cooling systems that JWST will. By the time that a theoretical secret optical JWST-KH were retrofitted with new mirror designs, new sensors, and new active and passive cooling systems for a public infrared science mission, you basically have a completely redesigned satellite.