The bipartisan immigration bill before the Senate could cost a whopping $6.3 trillion.

At least that's what the conservative think-tank the Heritage Foundation is claiming in a new report released Monday. 

The study by the group opposed to immigration reform is sure to draw fire. Already, the libertarian-leaning Cato Institute has decried it as "fatally flawed." Critics say it doesn't measure economic boosts from the bill.

The Republican-led organization says the costs come from more than $9 trillion in government benefits to newly legalized immigrants over their lifetimes — only partially counterbalanced by $3 trillion they would pay in taxes.

During Congress' last immigration debate, in 2007, a Heritage report said the bill under consideration at the time would cost $2.6 trillion. The conclusion was criticized, but carried weight with Republicans and the bill was defeated.

“Amnesty is unfair to those who come here lawfully and those who are waiting,” former Sen. Jim DeMint (R-S.C.), Heritage’s president, said during a presentation Monday. “It will cost the American taxpayer trillions of dollars over the next several decades and it will make our immigration problems worse.”

But Alex Nowrasteh of the Cato Institute said the new Heritage study has issues.

"They do not consider that increased legal immigration will increase the size of the U.S. economy and thus increase tax revenues – contrary to an overwhelming contrary consensus in the economics literature,” Nowrasteh told Politico.com.

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