when you think about it on a one time only basis , perhaps...
<I'm cutting out some math>
the week is over and you still have your robots to work the next week.
think of the savings to farmers and the savings to the environment.
and maybe you could rent the robots instead of purchasing them.
and the provider handles the upkeep of the robots.
You've got the costs of labourers down, although your $7/hr is unlikely. Most are paid by what they produce/pick; not by hourly wage.
But you've ignored the other half; the cost of the robots. I have no idea what such a unit would cost, but given that the simplest of modern manufacturing robots run over $10K each, and a farming robot would have to be far more complex (needs high dexterity to avoid damaging the crop, visual recognition systems - something I have some experience with, and trust me it ain't cheap or easy, must be mobile, etc), I'd be guessing you're talking about machines in the hundreds of thousands of dollar range (equivalent to a combine, btw, if your talking the sub-$250K range).
There is also the issue of multi-use. A laborer can pull weeds, pick produce, remove pests, apply fertilizer, water the fields, preform repair work, plant seeds, setup bird/shade netting, etc. Meanwhile, most farm equipment (and thus, presumably, most farm robots) can only do one or two functions. Ergo, your one laborer may need 2-3 robots to replace them.
And then you have operational costs (fuel, repairs, etc), maintenance, mandatory warranty work, and the wages of highly trained (and thus expensive) operators, etc.
And the last problem being the issue of farming being a seasonal activity - during which time your robots sit idle, but still cost you money (be it storage fees, winterizing expenses, or whatever).
One day I'm sure the technology will develop to the point where these things will be possible in a price range which is competitive. But given the state of robot tech today, and the relative inexpensiveness of seasonal laborers and students, we're a long ways off.