There is one true way to measure this, you may be offering skills but you are selling your time.
100,000 / (5240) = $48 p/h (8 hours a day)
100,000 / (5260) = $32 p/h (12 hours a day)
You are actually costing yourself $320 ($16600 pa) a week by working and extra 20 hours.
60,000 / (5240) = $28 p/h (8 hours a day)
60,000 / (5260) = $19 p/h (12 hours a day)
You are actually costing yourself $180 ($9360 pa) a week by working and extra 20 hours.
I understand this is a fairly black and white way of looking at it, and you could easily say hey I am earning 100K and happy putting in the time.
Now if you take Gary's V point of view on an 18 hour day
100,000 / (52*126) = $15 p/h (18 hours a day).
You want to make sure you get a return on your investment, and as an employee you probably never will. However a business person, I agree these hours may be beneficial.