We definitely owe them enough money that they could comfortably fill the payment gap to their next job (which might be never, meaning enough money to pay for inflation-adjusted lost income during the entire remainder of their career).

But in the name of pragmatism, it is unlikely that whistleblowers would blow the whistle on a purely economical basis if the reward wasn't higher than that.

Then we should also pay whistleblowers who report environmental issues a lot of money. I don't think that's the case.

If someone saves you a billion, what's $100m?

What is society but our debts to eachother

Considering that money came out of the fine that their employer paid, I don't see a problem.