That figure comes from Eric Jackson, a manager at hedge fund SpringOwl.
On Monday morning, Jackson sent a 99-page presentation to Yahoo's board of directors outlining his case for why Yahoo should drop Mayer as CEO and find new management.
Mayer's total compensation, if she stays for a full five years at Yahoo, will be $365 million, Jackson says in his presentation, citing SEC filings.
That's assuming that the company's stock at the end of those five years is at $40.34. The $365 million figure includes the one-time $14 million "make whole" award payment she will receive to compensate her for the stock she walked away from when she left Google.
The vast majority of her pay isn't tied to Yahoo's performance as a company - it's tied to Yahoo's stock price. Only about 3.3% of her projected $365 million compensation package is tied to Yahoo performance, and that's the company's annual target bonus of $2 million.
Mayer's total compensation, including cash and stock, increased 69% to $42 million in 2014, according to Yahoo's proxy statement filed on Wednesday. Mayer total compensation in 2013 was $24.9 million.
Mayer's base salary was the same in 2014 as the year before: $1 million. The increase in her total compensation was due to Yahoo's stock price, which has more than doubled since Mayer took the reins in the summer of 2012.
Mayer received stock awards in 2014 valued at roughly $11.8 million and option awards valued at $28.2 million.
With Yahoo's business languishing, and a new complex plan to spin the internet business into a separate company, there is increased speculation about how long Mayer will last in her role as CEO.