There is no "if". Obama announced his legalization plan on Spanish TV TO GET THE WORD OUT to the south of the border crowd. He wants to flood the borders and then, as you state, legalize millions more.
By: DrJohn
http://floppingaces.net/2014/11/23/the-tab-for-king-obamas-amnesty-decree-it-could-be-6-trillion/
The typical unlawful immigrant is 34 years old. After amnesty, this individual will receive government benefits, on average, for 50 years. Restricting access to benefits for the first 13 years after amnesty therefore has only a marginal impact on long-term costs.
If amnesty is enacted, the average adult unlawful immigrant would receive $592,000 more in government benefits over the course of his remaining lifetime than he would pay in taxes.
Over a lifetime, the former unlawful immigrants together would receive $9.4 trillion in government benefits and services and pay $3.1 trillion in taxes. They would generate a lifetime fiscal deficit (total benefits minus total taxes) of $6.3 trillion. (All figures are in constant 2010 dollars.) This should be considered a minimum estimate. It probably understates real future costs because it undercounts the number of unlawful immigrants and dependents who will actually receive amnesty and underestimates significantly the future growth in welfare and medical benefits.
The most recent studies after the fact, concluded that Obamas unlawful Amnesty will cost over $6 trillion just regarding Social Security and Medicare bankrupting both.