The study from the Republican-led group opposed to an immigration overhaul is sure to come under fire. Already, the libertarian-leaning Cato Institute has decried it as "fatally flawed." Critics say it doesn't measure economic boosts from the bill.
Heritage 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.