Just stumbled across this 2005 report from the Center for American Progress. "Deporting the Undocumented" puts a price tag on kicking all those illegals out of the country thus:
Using publicly available data, we estimate the costs of a mass deportation effort to be at least $206 billion over five years ($41.2 billion annually), and could be as high as $230 billion or more. Spending $41.2 billion annually would exceed the entire budget of the Department of Homeland Security for FY 2006 ($34.2 billion) and more than double the annual cost of military operations in Afghanistan ($16.8 billion).
Whole thing here.
In its May 25-31 issue, The Onion came up with cheaper ways to prevent illegal immigration, including "fill Rio Grande with sharks, making sure motorcycle ramps are on U.S. side"; "make U.S. jobs less cool and fun"; "place 'Welcome to Canada' signs at Mexican border to confuse illegals into walking south"; and "invent legend of Border Chupacabra, which eats anyone not in possession of valid U.S. passport."
Whole Infographic here.