The cost to recruit, background check, train line staff and supervisors is enormous; the salary and benefits would be huge and the time to do all of this would be years, not months. Trump is a demagogue, what he promises is impossible; he is also a charlatan for he has no idea how stupid his promises actually are.
BTW, each deportee is entitled to legal counsel and due process, which takes time; the detention of so many people requires lodging, feeding, medical care and supervision. Thus more administrative judges will need to be hired, more defense lawyers and more US Attorneys, more US Marshals and the care of children, many of whom will be US Citizens will further impact the final cost, in terms of dollars and human misery. Only callous jerks and fools believe Trump and hope he pulls off the impossible.