Ir will cost more at first sure, but as news spreads that the Obama ‘catch and release’ program has been ended it will drastically reduce the number of those wanting to cross the border illegally.
Obama’s policy to release everyone into America after capture as long as they claim they are being persecuted in their home nation had spread like wild fire and it drew in more and more illegals. As a side effect those picked up were processed as refugees, not normal illegal immigrants.
Mandatory detention will be a huge deterrent, just like building the wall or more/better fencing. Stricter deportations of criminals etc is all a deterrent. The idea is to get the word out to the world that the open door policies to anyone who can walk over the border that Obama advertised is now over.