In mid-August 2013, Carl Icahn made headlines by announcing over Twitter that he had taken a $1.6 billion stake in AAPL, claiming that the company was significantly undervalued. Over the course of the proceeding months, Icahn put forth his suggestions about what the company should do with its massive hoard of cash to return greater value to shareholders. Among those suggestions was an additional $150 billion of share buybacks and an increased dividend.
When AAPL failed to impress investors with its revamped 5S and 5C iPhones in September-2013, the stock fell over $56 (split-unadjusted), or 11%, from $506 (9/9/13 close) to $450 (9/16/13 close). Through that turmoil, Icahn bought more stock, increasing his stake at a per-share price between $465 and $467.
Icahn continued to lobby for the company to return additional capital to shareholders and even had dinner with TC to express his viewpoints. Finally, in April-2014 AAPL announced that its board had authorized the expansion of its capital return program to over $130 billion by the end of calendar 2015 - the largest capital return program ever on record. Icahn acknowledged that the expansion of the capital return program was good enough for him and most expected him to exit his position, with a healthy, yet modest gain. But, he didn't....
As a show of faith in the company, its management, and his continued belief in the enterprise's under-valuation, Icahn not only kept his stake, but expanded it further. On a split-adjusted basis, it is estimated Icahn currently holds 52.5 million shares of AAPL, which equates to about 0.9% of the company's outstanding diluted share count (6.05 billion).
By using different data points and information put forth by PED, I estimate his average, split-adjusted cost basis to be $83.80 per share - in reality, I think it's actually lower. As of Friday, August 22, when AAPL closed at $101.32, the estimated value of his position is $5.319 billion, equating to a pre-tax gain of $919.3 million. Additionally, he has participated in AAPL's dividend payout, which is estimated to add another $78 million in cash. In total, his returns look to be about $1 billion ($919.3 million in stock price appreciation and $78 million in cash dividends) in a bit over a year - a return of roughly 23%.
While Icahn receives much criticism for his tactics to enforce change within companies he believes are undervalued, he didn't achieve a net worth north of $25 billion by picking the wrong horses to ride.
Update: On April 28, 2016, Carl Icahn announced that he had exited his position of ~53 million shares of AAPL (~$5.025 billion) citing concerns about the company's market position in China. As of the close on May 5, 2017, had Icahn held onto AAPL, his stake would be worth ~$7.894 billion. He left ~$2.869B plus quarterly dividends on the table.