Hi Dan,
I like where you're going with that, but I see two problems.
1: In many roles, and in many organizations, it's difficult to quantify the value an employee delivers (particularly since it can vary greatly from one employee to the next). Unless you're talking about a project with a multi-million dollar budget, it may not be worth the time and effort to calculate with that level of detail.
2: Your math is off. In your example, the working employee produces a net value of $100/hour, while the non-working one generates a cost of $50. That's a difference of $150/hour, not $200. In other words, the employees are being paid the same, regardless of activity. The only difference is the output.
Cheers,
Simon