[DuncanWaring]

Are Americans willing to pay $4 instead of $1 for a head of lettuce?

This is specious, steaming crap.

It infers that the labor cost of a legal worker would be $3 per head.

If we assume a "burdened" labor rate of $20 per hour (including taxes, unemployment insurance, etc), if the picker is picking one head of lettuce a minute, that works out to 33 cents per head. And I suspect the actual rate of lettuce-picking is far faster than one head per minute.

[Celtman]

if the picker is picking one head of lettuce a minute, that works out to 33 cents per head.

      LOL!  Amazing how enlightening a little grade school arithmetic can be. 
      And, increasing legal wages would improve the Social Security situation ...