It is well worth the hit to the treasury for what we would save in welfare and legal costs. Don't announce the incentive program until after Trump takes office and gets congressional approval. My guess is that four to five million would be making plans to go home the day after Trump is elected and 80% of those would be gone before the program is even announced.
Finally, when we are about halfway to the goal, let's say five or six million illegals remaining, then announce that the cash incentive has been reduced to $100 plus a Trump administration version of amnesty.
This time amnesty would mean that anybody who self-deported or signed up to deport through the incentive program would be eligible to apply for legal re-entry.
Anyone remaining after x number of months would be rounded up, finger printed and deported with no rights for re-entry.
Here is a great article from Christian Science Monitor 2006 that describes Operation Wetback as I remember reading about it a few years back on Wiki. Wiki has since evolved, but this CSM article is pretty good-
http://www.csmonitor.com/2006/0706/p09s01-coop.html