That 0.5 billion (which to be clear, is a number I made up) won't guarantee that JWST will be fixed. The "fixer" spacecraft could end up failing itself. Or it could discover new problems that it's not equipped to handle.

Also, congress would be extremely critical of NASA if JWST fails. They would not be excited to shell out another $0.5 billion for a chance to fix it.

Not to mention that we don't even currently have a real capability to repair the currently malfunctioning Hubble telescope anymore. That was designed for maintenence from the now non-existent space shuttle in mind. Trying to accomplish the same with the Dragon would be unknown territory and it seems that there is little desire from both NASA and congress to even bother.

JWST in comparison is a far trickier and complicated beast to tinker with. This is the biggest reason why they are so paranoid about any fault before orbital launch. It would be all but impossible to service it - on both a technical and political level.