If local tomatoes become too expensive then the supermarkets will just import them from third world countries. That's what happens over here.
Actually if trade barriers are adjusted so that poor countries can receive fair trade deals then less people might stop trying to enter rich ones. Just a thought.
Importing food is not necessarily the only option. Especially for cheaper food.
For decades the US has been paying farmers NOT to grow food. They justified it with a "conservation" angle by planting trees instead. The last big farm bill in 1980s was during the first generation of tree huggers and a lot of farm forclosures(I think Farm Aid was one of the first big celebrity benefit concerts). Billions are paid to farmers NOT to grow food. If they're growing anything the land isn't a dirt field. Think if billions of dollars of food came on the market. It would add choice and competition to help keep prices down. And if they were to SECURE the border it would tougher to get illegal labor to work the farms. Smaller farms would require fewer employee but better paid employees. Many want those kind of farm subsidies stopped.
http://www.washingtonpost.com/wp-dyn/content/article/2006/07/01/AR2006070100962.html
http://tbo.com/news/nation-world/20...-end-paying-farmers-not-to-grow-cr-ar-151982/
An attempt was made this year to end the subsidies but they were repackaged in a January Farm Bill.
http://www.nytimes.com/2013/07/20/business/richer-farmers-bigger-subsidies.html?pagewanted=all&_r=0
Personally I think the big corporate farms and food producers a little more than something to do with this.
http://www.organicconsumers.org/ofgu/lionsshare121304.cfm
But even if there is extra food that wouldn't sell some of it could exported or donated. In the 1980s a lot of farmers had trouble but it's almost 4 decades later. I think we could export more than ever before. I'd like to see many more smaller farms and farmers. Ending subsidies must coincide with securing the border.