In my experience around both courtrooms and university professors from a variety of fields, they tend to fall all over themselves leaping at the chance to be expert witnesses.
It's a LOT of money for very little work, typically way more than they'd make on even a typical Ivy League full professor's salary or an academic book deal. And yes, I am including putting together a fully researched report on those findings as part of that work. Even in highly-compensated fields like CS, law, or business, the number of folks, even "titans in the field," whose salary calculus would make it sensible to pass up $2M, or even a tenth of that, for a week's part time work is so scant as to be non-existent as a practical matter.
The places where they don't tend to be willing to play ball are where they are asked to testify in a way that obviously undercuts their own research, but most experts are savvy enough to couch and qualify their testimony in ways that avoids this pitfall.