The cost - in freedom (already endangered by the looniness of gitmo, the Patriot Act, and warrant- free wiretaps) and in cold hard cash of rounding up 12 million folks (or more) to buy them ice cream cones, let alone push them through the court system while imprisoning them and then out the door to the relevant countries - is beyond what we can afford on either front as a nation. Right or wrong, it's wwaaaaay outside our budget.
Current immigration law is subjective and whim-driven, with a bit of dumb luck thrown in for spice. Before we start hurling folks out of the country, perhaps we need to take a long cold look at making sure the law WOULD throw the right folks out rather than folks who have every reason to believe they are legally in-country, or even citizens.
Consistency, objectivity, and logic would all be nice touches before we go on some grand campaign for deportation.
Finally, securing the border should and must be a separate issue from the issue of "how do we deal with folks already in country of questionably legal status"?
Frankly, I'd not object to mining the borders and running checkpoints at said borders and ports of entry - problem is, we'd still have the coasts.
The only reasonable course I see is to secure the borders, and then start processing those folks already within the borders into three categories or paths:
1) Legal Residence - felony free, gainfully employed or familial obligation...goal: citizenship.
2) Temporary Worker - 3 year non-citizenship temporary worker visa, upgradeable at U.S. Embassy in country of origin.
3) Immediate deportee - felon, chronically unemployed, etc.
That, we can do. Folks willing to work hard for citizenship, we can afford. And manual labor, we need.
That's just my take.