It's certainly questionable, I didn't mean to imply it was more than a rough guess. It's true, professor jobs at research universities have an "interestingness" premium, so part of the multiple comes from that.

On the other hand, lack of external employment options makes tenure MORE valuable, since it raises the risk premium the teacher would demand to quit the tenured job.

Since my financial math is rusty, I won't try to compute the actuarial values of these revenue streams, however.