This week, the Washington debate over immigration reform is turning to a critical question – what will it cost? 

There is, it may seem, a different cost estimate to suit every voice in the debate. The Heritage Foundation released a report Monday attempting to calculate the cost of public benefits used by immigrants (totaling over $6 trillion according to the study), leaving out any increase in economic growth. Doug Holtz-Eakin, a conservative economist, released a paper last month taking the opposite position – increased growth from immigration, he said, could reduce the deficit by about $2.5 trillion. 

The final word on the question will come when the Congressional Budget Office and Joint Tax Committee release their official scores on the proposed immigrant bill. In 2007, CBO estimated that over ten years immigration reform would cost $43 billion to implement and increase spending on social programs by $23 billion, and add $48 billion in new revenues. The estimated net effect on the national debt was small.

But often lost in the noisy back-and-forth over different cost estimates are five important underlying concepts that will determine the eventual cost of immigration reform:

1. The Starting Point For Any Fiscal Analysis Should Be ‘Compared To What?’

Whether or not we reform our immigration system, the 11 million undocumented immigrants who are already here are working, paying billions of dollars in taxes, and consuming public benefits like Medicaid-funded emergency healthcare. Mass deportation of these immigrants would be a costly, infeasible undertaking – a 2010 Center for American Progress report estimates that such a step would cost about $285 billion over five years, and that’s before factoring in the economic impact of losing millions of workers.

With deportation not a viable option, policymakers focused on the bottom line should be asking how legalization would change the fiscal costs and benefits caused by immigrants – not just adding up the costs of public services immigrants will use in the future. In budget parlance, the costs of immigration reform should be evaluated against a ‘baseline’ of continuing the status quo.

In the absence of reform, our current system would continue to deteriorate. Many of the costs imposed by undocumented immigrants (most significantly emergency healthcare) will continue to rise, while key benefits (such as increased productivity from rising education levels) won’t materialize. And increased skilled immigration, which unambiguously increases economic growth and has a limited impact on government spending, is unlikely to occur without comprehensive immigration reform.

2. Legal Status Will Make Previously Undocumented Immigrants More Prolific Taxpayers

Uncertain legal status makes it difficult, if not impossible, for undocumented immigrants to make basic investments in their own productivity. Almost half of working-age undocumented immigrants lack a high school degree, seriously limiting their ability to contribute to the economy (the wage premium for a high school degree is a whopping 40%). Undocumented immigrants also miss out on the benefits of ‘learning by doing’, because the constant risk of discovery and prosecution leads to high levels of turnover, preventing immigrants from staying in the same job long enough to undergo both formal and informal training.

In fact, a Vanderbilt study of Latin American immigrant men who received legal permanent residence in 2003, compared to control groups that were continuously legal or undocumented, found that the group that was granted legal status was the only one to experience a “structural improvement” in labor market outcomes. Increased returns to education, the study found, boosted newly legalized immigrants to more productive (and better compensated) occupations.

Most economic analyses of immigration reform focus on the additional economic output generated by new immigrants – but the (admittedly harder to measure) productivity increase for the 11 million undocumented immigrants already in the country could also have a significant fiscal impact.

3. Immigration Probably Won’t Spark A Rising Tide Of Welfare Costs

The 1996 welfare reform legislation barred non-citizens from using major means-tested welfare programs, including food stamps, Medicaid and traditional welfare. Some analysts – including the authors of the Heritage ‘cost of amnesty’ report – have said that a path to citizenship for undocumented immigrants will create millions of new (and costly) welfare recipients. But studies of naturalized citizens show that immigrants are unlikely to seek citizenship just to obtain welfare benefits. And costly retiree programs, namely Medicare and Social Security, are only open to those who have been employed legally in the U.S. for at least 10 years, a requirement that is not waived for immigrants.

According to a recent paper by the conservative CATO Institute, low-income immigrants tend to use means-tested welfare programs at a lower rate than natives, and the cost of their benefits is substantially lower as well. This difference isn’t just driven by access to benefits – benefit use by poor immigrants was lower even before the 1996 welfare reforms.

4. Young, Healthy Immigrants Can Lower Healthcare Costs

Undocumented immigrants can help lower system-wide healthcare costs, following the same logic in the President’s healthcare reform law – adding more people to the insurance pool spreads risk and costs more broadly, and insurance coverage generates savings by replacing expensive emergency care with cheaper, preventive care. This case is even stronger for immigrants – who are healthier, and use healthcare less, than the general population – than it was for the previously uninsured population targeted by the Affordable Care Act.

In its 2007 immigration reform analysis, CBO pointed out that multiple sources have found that undocumented immigrants tend to be younger and healthier than the rest of the U.S. workforce. A 2006analysis found that only 19% of recent immigrants have reported chronic diseases, compared to 38% for U.S.-born citizens.

5. Without Immigration, Social Security Would Already Be Bankrupt

Even though they don’t have legal status and in most cases can’t claim Social Security benefits, undocumented immigrants have contributed up to 10 percent (or between $180-300 billion) to the Social Security trust fund. In 2010, the SSA’s Chief Actuary Stephen Goss has said that if not for the millions of illegal workers paying into the system Social Security would have “entered persistent shortfall of tax revenue to cover payouts starting in 2009.”

Immigrants Will Drive Labor Force  GrowthThere are already about 200,000 Baby Boomers exiting the labor force each month through retirement, a figure which will only increase in coming years. The Census Bureau projects that in  2050 there will be almost 90 million seniors, more than double the 40 million seniors we have today. Whereas 40 years ago there were 5 workers per retiree, today we are down to just over 3, and by 2025 trends suggest there will be just 2 workers for every 1 retiree. Without immigrants and their children, the nation’s labor force would begin to shrink around 2015, worsening demographic-driven budget problems caused by soaring retiree beneļ¬t liabilities.