Nah..!
He needs to ALSO levy some tax on Western Union remittances for EACH illegal caught, to offset the US costs of their capture, etc.
Remittances are juuuust behind oil for Mexico in terms of national income: they have NO interest at all in curtailing illegal immigration.
In fact, the Mexican support for it’s citizens radically escalate AFTER they have left:
To the Mexy Gummint people STILL THERE are annoying expenses, people who have left are pleasant INCOME.
That’s a great idea. For each illegal caught, confiscate remittances to Mexico. Make it proportional. Hit them where it really hurts.