It is an actuarial evaluation. They estimate the number of years until retirement, the number of years of survival after the retirement, and then do a weighted average of a few different scenarios to determine the present value of the future pension entitlements.
I've seen it range from 30k to 800k for a typical defined benefits plan. That is certainly not a hard range, it can be less or more. The pension can be split at the source though so it doesn't matter that much.