The purpose in finding out the costs of a data flow interruption is not to retrospectively determine lost money, but to figure out how much potential money will be lost in future data losses. This facilitates how much one should spend on current efforts to minimize data flow interruption.
First, one can either use the average hourly rate of all the employees, or use a distribution of population. The relevant population is not "all users" necessarily, but the users who are seeking data while the interruption is occurring.
If you are down t hours, then the cost of idle workers is t*(average hourly salary)
The next cost is the percentage of customer base lost to competition. This can be ignored if you are the sole solution.
The cost of reputation decrease, with respect to fewer future customers. Again, ignore if you are a monopoly.
Lastly, either the work hours spent recreating the data, or pessimistically, the hours lost during creation. These aren't the same if the lost data isn't recreated.