With the exception of emergency medical treatment, no illegal alien shall receive social services of any kind. No free housing or housing vouchers. No food stamps. No Medicaid. No financial assistance whatsoever. Withhold federal funding from states and cities that provide housing and other assistance to illegal aliens.
By doing so you will greatly reduce the cost of mass deportations, as many will choose to leave and the invasion will slow to a manageable trickle.
Make it possible for them to request assistance in leaving and provide them safe transportation back to the border. This will reduce the number of illegal aliens requiring deportation and save the taxpayer money.