Heritage Foundation President Jim DeMint on Monday said that he opposed a bipartisan plan for comprehensive immigration reform because "unlawful immigrants" could live "another 50 years" and take advantage of government benefits that they earned by paying taxes.

At a press conference on Monday, the Heritage Foundation released a report that claimed the immigration reform bill offered by the "Gang of Eight" senators would cost Americans $6.3 trillion.

"The number over the 50-year lifespan of an amnesty for unlawful immigrants, it's $6.3 trillion to the American taxpayer," DeMint told Fox News host Martha MacCallum prior to the press conference. "And we know over time that this is going to increase more debt, increase taxes. That has a depressing effect on our economy. And we know that unlawful immigrants -- once that they have amnesty -- are going to replace the jobs of many Americans and depress their salaries."

"So there is no way you can look at this and conclude that it's good for the American taxpayer, and that includes immigrants who are here lawfully."

MacCallum noted that most benefits would not be available to immigrants for 13 years under the proposed plan.

"I'll believe that when I see it," the former South Carolina senator quipped. "Even if they follow through, unlawful immigrants are already receiving many benefits. A lot of their children are legal American citizens, whether its public education or Medicaid. But if you just look at a 13-year window when the life expectancy of unlawful immigrants goes another 50 years and once they get on Social Security and Medicare -- I mean, the average cost of an unlawful immigrant is hundreds of thousands of dollars."

DeMint added that he was all for immigration reform as long as "lower-skilled, less-educated" immigrants were excluded from the plan.

"In 1960, the average immigrant had about the same education and skill level of an American citizen. Today, immigrants have -- they're four times less likely to even have a high school diploma. And now with all of our welfare benefits, the arithmetic for immigration is totally different."

Watch this video from Fox News' America's Newsroom, broadcast May 6, 2013.

(h/t: Twitter/@igorvolsky)