Well let's look at it this way.
Let's say every year 25,000+ people immigrate to the US from Mexico (very small estimate)
So every 5 years 125,000 immigrate here.
With a wall, tightened security, etc... Less than 10,000 people immigrate here illegaly. So every 5 years less than 50,000 immigrate here illegaly.
Now, back at my 25,000 people estimate. Let's say the average tax charge is $1000. This means that $25,000,000 is gone because of illegal immigration. This money lost is nothing compared to the real number. Every 5 years we lose $125,000,000 because of illegal immigration.
Imagine the good that could come out of 125 million dollars for the US. We could start paying back the national debt, we could also start helping poorer families.
http://www.cis.org/articles/2001/mexico/release.html
^^^For more stats.