Funny, but there is always talk about free trade here. And why are US veggies so cheap. Aside from them usually being these crappy and tasteless GMO varieties, I tell people that US produce is subsidized. Not in an obvious way, so technically they are not violating trade agreements. But in two big ways:
1- Water. California is tearing through water with incredibly wasteful practices. Much of the agricultural land there that feeds the nation is literally desert. They flood it with poorly developed irrigation that means most of the water is lost to run off and evaporation. Meanwhile the sources are drying up. It's criminal abuse of an important resource, and nobody is even discussing the problem. But, at least for now, it keeps the fruits and veggies cheap.
2- Illegal immigrants. They work cheaper than locals would. Without the flood of illegal immigrants, the US would have to pay more, or invest more in machinery and robotics. This will come to an end too. Australia and New Zealand manage to export fresh produce with massive illegal labor. Maybe the US should try that too.
Both practices are unsustainable - and it's just a matter of time.
I agree with the comments above, as well. Forget the fence. Just enforce the laws against hiring illegal workers. Make the fines hurt. And stop providing social benefits to illegal immigrants - it's insane.
Once the jobs and the benefits dry up, so will the influx. Some will still come, but they will be motivated people who will make their own way in the country, support themselves, and become useful citizens. Either that, or they will be criminals and drug dealers.