How would Mercedes be performing in 2012 if Hamilton was already Rosberg's teammate?
I looked at Hamilton, Button, and Rosberg. I took each driver's fastest time from any of the three qualifying sessions at each race. Then I took the gaps from Hamilton to Button, and from Hamilton to Rosberg. For each of those two comparisons I dropped the 3 best and 3 worst gaps. There have been 14 races in 2012, so this leaves 8 typical gaps between Hamilton and Button, and 8 typical gaps between Hamilton and Rosberg. I took the average of those 8 gaps for each comparison.
Hamilton was 0.36 seconds per race better than Button. Hamilton was 0.43 seconds per race better than Rosberg.
I will make a reasonable assumption that Rosberg is as good as Button. Button is damn good and estimating Rosberg as his equal is doing Rosberg no injustice. The difference between Button and Rosberg is only 0.07 seconds per race (0.43 - 0.36 = 0.07).
This implies the McLaren is 0.07 seconds better than the Mercedes, but Hamilton is 0.36 seconds better than a grade-A F1 driver such as Button.
In other words, around 80% of the difference between Hamilon and Rosberg this season is due to Hamilton, and 20% is due to the car and team.
McLaren has to find 3 or 4 tenths to replace Hamilton.
Mercedes only has to find 1 tenth (0.07 actually) and they'll be championships contenders.
I think McLaren will come to realize that Hamilton was the cheapest three or four tenths that money could buy. How much money will they have to spend to come up with 3 or 4 tenths from the car at every race? They can't-- the RRA won't allow it. I can't help but think that McLaren simply screwed up the negotiations. Do you really think McLaren is happy with a $12 million per year savings and Perez in the cockpit?