If she gets Yahoo back onto the expressway, she will be responsible for the success, although all the employees working together will be implementing the success.
You may not like that that's the way it works, but I think it's impossible for the CEO to fail to have a huge impact unless the company's run terribly. I also think it should be trivial to see that, given that the CEO has such a huge impact on the direction the output of this large number of employees takes, that the CEO's actions have an enormous impact on the value the company provides, and consequently the profit the company derives.
If the CEO of Yahoo decided that their best move was to pursue 1990s era AltaVista style search, combined with the idea of 'being a homepage' (which afaict is their current strategy, plus destroying good products), I think it'd be hard for you to NOT recognize that they would be destroying value in the company at a much higher rate than $150M/year. The CEO, personally, would have that impact. If the employees were more efficient at their jobs, or less efficient, it would have little impact (rounding error) in the overall impact of this policy pursuit.
CEOs are the company, for many purposes. Why shouldn't they be paid accordingly?
Ergo, CEOs actually do have quite a bit of value.
Only the CEOs and their hangers-on actually believe that.
One way to answer your question, would be to say: It seems the shareholders of Yahoo! are willing to pay a fair commission to someone in exchange for an increase in the value of their stock (with optimism that Mayer's leadership will result directly in this happening).
But that's not actually value generated. It's value captured from the grand casino of the stock markets. Yahoo! itself is not holding 1 billion more dollars at the end; Yahoo! shareholders are collectively, all tens of thousands of them, holding 1 billion in potential gains should they all sell their stock in some way that won't crash the stock price from the sudden sell-off.