I am likely the only person on this thread that has picked grapes. I buy 800 pounds each year from a local vineyard here in central VA. A good friend over in WV and I make wine every year.
After helping some migrant workers pick for a morning I will say that they work their butts off. Put me to shame with their speed picking (Of course I am in my 60s and have essential tremors, so they had some edge :-)
My point is that I imagine it would be a rare high school or college student who would be interested in picking grapes for even $20 an hour. Some might start, but would likely walk away after a couple of hours. It may not sound like hard work, but it is.
I am sure that at some point harvesting equipment will be designed to pick grapes and this whole issue will be history.
There are already machines to pick grapes:
http://www.airfieldwines.com/blog/2010/05/11/the-scoop-on-machine-harvesting-wine-grapes/