10 billion is totally acceptable considering how much we spend in Afghanistan and Iraq every month.
I've long said that drones, cameras/lazercams and more men/women in jeeps are the most effective tool for stopping illegal immigration.
All of you "build a giant fence" people are incredibly naive.
Take it from me, I am half Mexican. It can only keep people back from so long before they tear down little chunks of it. It's a huge waste of money. It's about as useful as just building huge amounts of fence around a prison of 10K and only having 1 guard. A modest fence and 100 guards are much better.
P.S. By 2035 if growth continues on a similar path (and it likely will for Mexico) by 2035 it will not be economically beneficial for poor Mexicans to come here looking for work. http://en.wikipedia.org/wiki/BRIC#BRIC_in_future
The gap will have closed by too much. Sure people will still come here to study, and people who come here because they want to live here rather than than being poor, but it will not be useful for poor people to come. They won't be able to send enough money home at the end of the month for it to be worth it.
... I'm not entirely sure why you feel the need to lump me in with the "build a wall" people. Building a wall simply wouldn't work. But with this thread, I imagine that it can indeed be done albeit with lasers instead for a relatively cheap amount with a 100% success rate.