I don't understand how the cost of identifying them and sending them back could be more than setting up a regulated program, identifying them, putting them in a regulated program, keeping track of them and then sending them back after their time is up.
As someone else said you also have the cost of schooling, hospitalization, medical care and other welfare services for them. As long as they are here they have more time to make babies and once their newborn is automatically granted US citizenship, they are basically here to stay. At the very least, we should change the law that gives any child born in the US automatic citizenship before you go ahead with this idea.
I think it would be cheaper in the long run to concentrate on the borders now and then just send them all back. After the borders are secure you can talk about extended visas for work programs for people that enter our borders LEGALLY.