The Department of Homeland Security has at least ten unarmed Predator drones, costing approximately $62 million a year. Source

During his campaign, Trump initially said he could build a wall for $4 billion and later estimated $6 billion to $7 billion. In April 2017, he put it at $10 billion or less. Source

So for the price of building a wall, even at his conservative price of $6 billion dollars you could have over 20 drones, literally 1 drone for every hundred miles of border, that has the ability to continuously track, monitor, follow and alert incoming or fleeing migrants well before and after the wall for 48 years.

This does not even include maintenance cost of the wall.