It wouldn't double the cost assuming they do it with even a modicum of efficiency and intelligence. And hey - assuming that the launch goes smoothly, you can get double the benefit
Any space telescope with a mirror, is a different story, e.g.
https://science.nasa.gov/mission/webb/webbs-mirrors/
The RST has an 8ft mirror. The mirrors have to be ground and polished in a clean room to tolerances that are the highest in the world. I vaguely recall it's a square law or worse - for radius-to-time-to-polish. So you're paying for time, and you're paying for the expertise of one or two custom built places that can manufacture them.
https://en.wikipedia.org/wiki/2012_National_Reconnaissance_O...
When I worked for Mars Global Surveyor, one of the museum-pieces in the lobby was a glass case with the Viking spectrometer flight spare.
The flight spares were exceedingly useful when the spacecraft had some kind of trouble during its mission, because you could mock-up the flight spare in a lab and replicate many situations very closely, and propose solutions based on the mockups.
It’s true, but the factor is ten, not two.
And then they won’t even use any of the things they overpaid for!
Bon Voyage, RST!