He's overpaid because he makes an amount of money equivalent to about one-third of Kentucky's entire basketball revenue. One out of every 3 dollars the basketball program generates, in any given year, goes into Cal's pocket. As a percentage of total revenue, that just seems astronomically high. As a point of comparison, less than one out of every 10 dollars Alabama football makes goes into Saban's pocket. Saban has had far more success at Alabama, and in a higher revenue sport, than Cal has had at Kentucky.
The single biggest reason the $9m figure is so high isn't because it's a knock on Cal, but because he's the basketball coach. I know most of Kentucky's athletic identity as an institution is wrapped up in basketball, but even at Kentucky, basketball brings in less revenue (and profits) than football. Even at a school like Duke football revenue almost equals basketball revenue. Any school, even a "basketball school," would rather be better at football than basketball. It simply brings in more money, and a school stands to make a lot more money via incremental improvement in the football program than the basketball program.
If Saban made that same percentage of Alabama's football revenue, he'd make about $35m/year. Now, I've argued for years that Saban is underpaid, but even he wouldn't be worth $35m/year.