CNN recently reported that it would cost approximately $100 billion to remove all 12 million people in the U.S.  "An ICE [Immigration and Customs Enforcement] spokesman later said the $94 billion did not include the cost of finding illegal immigrants, nor court costs -- dollar amounts that are largely unknowable." 

Another unknowable number, and one most relevant and material in my opinion, is the actual number of undocumented workers in the U.S. - some have put this number as high as 20 million, as opposed to ICE's estimate of 12 million.  Secondly, the cost of finding and processing/prosecuting these undocumented people is 'unknowable' according to ICE, and hasn't been factored into the $100 billion amount [which only includes ICE personnel costs, room and board in a detention cell for 32 days as well as transportation back to the person's home country].  These two additional costs would easily dwarf the prison and transportation costs.

Collateral costs would be immense as well.  Employers would lose billions; so would the retail industry, housing market, auto dealers and banks, to name a few. 

Bottom line: the actual cost may be more than double ICE's figure, plus the aforementioned collateral costs.  Even assuming the money is made available, and assuming that the 12-20 million people can somehow be found, logistical bottlenecks in ICE manpower, court access and detention cells, as well as the grim reality of the true economic impact of such a move will preclude any serious attempt to mount a large scale removal program. 

ICE's cost analysis is not helpful, accurate or realistic.  They may just as well have announced the cost of a popsicle stick ladder to the moon without factoring in the cost of glue, and while pointedly ignoring the laws of gravity and physics.  We need to face reality and look to comprehensive immigration reform.