Legendary investor Carl Icahn today pledged $150 million to launch a new Super PAC designed to advocate for what he calls "desperately needed legislation" to address "the pernicious effects that are occurring and will continue to occur as a result of Congress’s failure to immediately stop so many of our great companies from leaving our country." This is the problem of so-called tax inversions, in which companies that are primarily based in the US shift their nominal headquarters overseas to reduce their tax burden. It's an interesting issue, a somewhat important one, and something that Congress should address.

1/2 I am starting a Super PAC with my initial commitment of $150 million to help end the crippling dysfunction in Congress — Carl Icahn (@Carl_C_Icahn) October 21, 2015

2/2 I have therefore sent the following letter to the relevant committees and leaders of Congress: https://t.co/EGcuHvzou4 — Carl Icahn (@Carl_C_Icahn) October 21, 2015

But peer into the details, and what Icahn is actually doing is pledging $150 million to the cause of helping Carl Icahn get even richer. As large a sum of money as $150 million is, Icahn's personal stake in the legislation at hand is even bigger.

Icahn is pushing a bill to create a one-time windfall for shareholders

The specific bill Icahn is advocating for isn't a narrow effort to address tax inversions; it's a broad effort to cut corporate income tax rates while generating a one-time windfall for people who currently own large blocks of shares in cash-rich companies. That's what Icahn is talking about when he refers to "passing legislation for international tax reform as outlined in the framework put forth by Senators Charles Schumer and Robert Portman, and supported by Chairman Paul Ryan, to fund the Highway Bill."

Here's how it works:

Right now, US-based firms have about $2.2 trillion in cash stored in their foreign subsidiaries in order to avoid paying US corporate income tax on it.

Schumer and Portman want to change the law in two ways to have less money stashed in tax shelters.

They want to levy a modest one-time tax on the $2.2 trillion.

After that, they want to impose a substantially reduced rate on future foreign earnings so as to increase the incentive to actually pay up rather than investing in tax avoidance.

Icahn stands to reap massive personal gains from this idea

Proponents of this plan hope that it would increase federal tax revenue. They also hope that by making it cheaper for companies to "repatriate" cash from their foreign subsidiaries that it would increase their incentive to invest money in the United States. But another thing that it would do is make it cheaper for companies to pay dividends to shareholders. Right now, if you are a company with profits stashed abroad you need to pay corporate income tax on those profits before you "bring them home" to pay dividends.

The single company with the most overseas cash is Apple. Icahn is one of Apple's largest shareholders. His big idea for Apple as a business is that it should pay more dividends.

So let's do the math:

Apple has $180 billion in overseas cash right now.

Icahn owns 0.5 percent of Apple, or $900 million in cash.

Under current law, Apple would need to pay $315 million in taxes on that $900 million before giving it to Icahn.

Schumer-Portman does not specify a number for the one-time levy, but a number in the 8 percent range would be sufficient to plug the hole in the Highway Trust Fund.

At an 8 percent rate, Icahn would save about $243 million in potential taxes on his Apple dividends. That — or even a slightly higher rate — would easily generate a healthy ROI for a $150 million investment in a Super PAC. And that's just from the one-time levy, just from Apple. Icahn presumably has other investments, and Icahn also would be able to take advantage of continued lower taxes on Apple's foreign profits.