There are many pros and cons to keeping Diablo Canyon running, especially cost for the latter.
Diablo Canyon has already tapped into a $1.1 billion grant from the Biden Administration to keep it from shuttering, however it still needs billions in repairs/retrofits to even continue running.
A report by the Environmental Working Group estimates the cost of keeping Diablo Canyon running through 2045 at between $20 and $45 billion:
https://www.ewg.org/research/outrageous-costs-deadly-dangers-real-risks-keeping-diablo-canyon-open