(Charles Dharapak/AP Photo)

When two titans like President Bill Clinton and Microsoft founder Bill Gates share the same stage, the subject inevitably turns to the political futures of their wives.

"I don't think Melinda is going to run for president," Gates joked today.

Clinton was even less decisive about the prospect of Hillary Clinton 's running for president in 2016.

"She hasn't mentioned it to me, either," Clinton said, speaking at the Peter G. Peterson Foundation's 2013 Fiscal Summit in Washington, prompting laughter from the audience.

"I don't know what's going to happen, but I know this: that is the worst expenditure of our time," Clinton said.

Since leaving her post as secretary of state, speculation has swirled that Clinton has her own presidential ambitions.

READ MORE: Is Clinton Dropping Clues to 2016 Run?

Bill Clinton said it is "frustrating" that the conversation has gone back to politics so soon after the 2012 elections.

"She's taking a role in the foundation, she's writing books, she's having a little fun being a private citizen for the first time in 20 years," Clinton said. "We need to be worried about the work at hand. All of us do. So whoever the next president is has an easier set of choices."

The wide-ranging conversation also delved into immigration policy, which comes to a head this week as the Senate prepares to debate a version of a comprehensive immigration overhaul bill proposed by a bipartisan group of lawmakers known as the " Gang of 8. "

Both Clinton and Gates slammed the Heritage Foundation study on the potential cost of immigration overhaul, which was released Monday to wide derision among conservatives and liberals.

The study predicted that the bill in its existing form would cost U.S. tax payers at least $6.3 trillion in the next 50 years.

Bill Clinton Tuesday echoed experts and immigration advocates who have said that the study's authors employed questionable methodology intended to take into consideration only the costs of immigration and not the potential benefits.

"For one thing, it shows only the down sides and the worst-case, downside scenarios with none of the upsides for immigrants," Clinton said. "Think about everybody who ever came to this country as immigrants and started a company or took a good job and built a family and became middle-class taxpayers; sent their kids to college.

"It's one side of the ledger and the worst-case scenario on one side of the ledger," Clinton said. "I'm for immigration reform because we need to stay younger than our competitors."

Gates, who is also the co-chair and trustee of the Bill and Melinda Gates Foundation, has advocated for an immigration system that attracts "high-talent" immigrants. He said the benefits of highly skilled and general immigration are no mystery.

"The economic benefits are very clear," he said. "The idea of someone showing that as a net negative, that is strange. We are the envy of the world, as the president says, because we have a growing population."

Former Republican senator and Heritage Foundation president Jim DeMint has defended his organization's study, contending that advocates for "amnesty" are hesitant to consider the policy's costs.

"No sensible thinking person could read this study and conclude that over 50 years, that this could possibly have a positive economic impact," he said.