Right, I suspect it will come largely as a hypothetical: This farmer (on a small scale) resolved to use all citizen workers, and to do so, he found he had to pay them this much. So that gives us a measure of how much more would have to be paid across the nation if all agricultural labor in the nation were done by citizens. And that would add X to the average person's price of groceries.
Problem is that the math is not so straightforward as "have to pay this much" if you want your hypothetical to be even related to reality. If it were a simple wage x vs wage y then you could maybe come up with something, but the more important consideration is conditions. What would it cost to create working conditions where American citizens can fill those jobs, whatever you use for wage y? In California you might find an American citizen who claims to be more than willing to do the job, even at current pay, which is not nearly as low as it is made out to be. But hiring that American citizen is like writing yourself an invitation to visit the California Board of Labor.
I once knew this guy who was a labor broker, and had the opportunity to explore this first hand, since he knew I wasn't a rat. I showed up at this parking lot before the crack of dawn and piled into a van with more guys than there were seats (illegal, and patently unsafe) and rode out to a field. We worked pretty much dawn to dusk with no guaranteed wage (illegal) picking cantaloupes (hard) and being paid by the bin. Breaks were neither scheduled nor encouraged (illegal), but since you were paid by the bin if you took a break it was at your own expense. Lunch that was probably not up to health inspector standards was provided along with a few coolers of water, but bathroom facilities consisting of 'giving back to the field' being all that was provided is also illegal. At the end of the day we piled back in the van for another illegal trip, then got paid in cash with no records kept (illegal) when we were put out in the same parking lot we started in.
At the time I had a state job where I got paid not a whole lot but far more than I really earned and still far less than a lot of my fellow state employees who had even slacker jobs than I had, and I made about half my usual day's pay. There were guys in that van who made more than three times what I made on the day, so they actually were making
more than I made at my regular job, for that day at least. There are probably plenty of citizens who would have been happy to have their day's pay, and maybe even a few who could develop the skills to do the work to get it. But the entire process would have to be totally restructured to make it even a consideration...and that restructuring would cost way more than would be reflected in 'wage x vs wage y.'
By the way, I was in good shape then and had a pretty 'physical' regular job, and I was totally wrecked by that one day. I couldn't have shown up the next morning for a million dollars, and the morning after that was even worse. Long term (and not terribly long either) I would guess that work would be crippling for some fair percentage of the workers. Any 'health insurance for your labor force' deal would have to charge an absolutely obscene premium.
