Transporting several millions of people yearly and cheaply(these people don't pay for extra leg room) is not really rocket science.

Right now it's so expensive because the government is as always disconnected from reality. They are deporting each individual on case by case basis using. air flights. The gov is even allocating one or two officers for each immigrant. Now consider boarding all the illegal immigrants on a big livestock carrier. It may take a while to reach its destination(s) but it will be cheap and surely the immigrants(at least most of them) would not try again the adventure. One big immigration camp in a single trip.