The comprehensive immigration overhaul being taken up in the Senate this week could cost taxpayers $6.3 trillion if 11 million illegal immigrants are granted legal status, according to a long-awaited estimate by the conservative Heritage Foundation.

The cost would arise from illegal immigrants tapping into the government's vast network of benefits and services, many of which are currently unavailable to them. This includes everything from standard benefits like Social Security and Medicare to dozens of welfare programs ranging from housing assistance to food stamps.

The report was obtained in advance by Fox News.

"No matter how you slice it, amnesty will add a tremendous amount of pressure on America's already strained public purse," Robert Rector, the Heritage scholar who prepared the report, said in a statement.

The study is already coming under criticism from some groups and economists who challenge its assumptions, claiming the legalization would help fuel economic growth. Heritage Foundation President Jim DeMint, though, defended the study ahead of its release Monday morning.

"There's no way you can look at this and say that it's good for the American taxpayer," he told Fox News.

The numbers could raise additional concerns for Republicans as a Senate committee prepares to consider the legislation later this week.

The comprehensive study, aside from looking at benefits, also factored in the cost of public education and other services like highways and police. The government is already providing some of those services to illegal immigrants, so the $6.3 trillion figure would not represent all new costs.

But most of that cost would be new spending, according to Heritage, as illegal immigrants gain access to additional government programs. The study acknowledges that, for a 10-year period, illegal immigrants seeking a reprieve would be barred from these benefits. After that window, though, Heritage forecasts the costs skyrocketing.

On an annual basis, the report estimates the cost will be $106 billion after the interim phase is over. In the course of their lifetime, the report estimates that illegal immigrant households would receive an average of $592,000 in government benefits.

The $6.3 trillion figure is based on what illegal immigrants would cost the government over the course of their lifetime. It factors in the expected $3.1 trillion in taxes they'd pay to the government.

Supporters of immigration legislation have been skeptical of efforts to assign a cost to the immigration bill. Proponents argue that the value of bringing millions of illegal immigrants out of the shadows and presumably into the taxpaying workforce is immeasurable.

Economist Douglas Holtz-Eakin, former director of the Congressional Budget Office, said the Heritage study ignores key factors like the possibility of illegal immigrants moving up the economic ladder.

"There's no upward mobility," he told FoxNews.com. "They're frozen" in low-paying jobs.

Holtz-Eakin said the estimate assumes "no American dream" for those who attain legal status.

Stephen Moore, an economist and Wall Street Journal writer, said many economists challenge the notion that immigrants are a net cost to the country. He told Fox News despite the Heritage findings, there are other studies showing the legalization will be an economic boon that could grow the economy -- in turn alleviating the country's deficit problem.

"You've got to look at both sides of the equation," Moore said. "Yes, the immigrants will use benefits, no question about that, but as they become more productive citizens and they come out of the shadows, a lot of economists -- myself included -- think they'll become more productive and they'll pay more taxes."

He noted many immigrants are entrepreneurial, starting businesses that grow the economy.

Sen. Marco Rubio, R-Fla., a key co-author of the legislation, has also stressed that illegal immigrants applying for legal status would not have access to federal benefits while they are in provisional status.

After obtaining a green card, they would still be ineligible for many federal benefits for five years.

Rubio plans to attend a meeting Tuesday in his office with more than two dozen conservative leaders who support immigration reform, a Republican planning to attend the meeting told Fox News.

The legislation also might not legalize all 11 million illegal immigrants. Some could be disqualified if they have a felony record or other problems in their background.

Heritage claims its estimate is on the conservative end.

"Those who claim that amnesty will not create a large fiscal burden are simply in a state of denial concerning the underlying redistributional nature of government policy in the 21st century," the report said.

Click to read the report.