So why didn't he advocate for leniency in the thousands of other instances where "it would've saved taxpayers about $x"?

He did. I read the book. One of his biggest legislative accomplishments before prison was reducing from felony to misdemenaor status cases where dads failed to pay child support (usually due to their not having work). Lots of guys were going to prison with felony records for the crime of being divorced and broke. Saving taxpayers money was a big part of the argument for getting this passed.

I haven't read the book but I suspect from the article that's one of its conclusions.

Would that help win an election?

Yes, I'm sure that sitting at home and getting to go out and coach baseball instead of being stuck in a prison had nothing to do with his reasoning.