It makes me wonder to what extent migrant labor features in the other parts of that apple's value chain, though.

Much of the price of that apple comes from the farmhands who loaded the truck, the truckdriver who drove it, the gas that the truck burned, the warehouse that stored it, the shopkeeper that stocked it, and the cashier that sold it. I don't know the prevalence of migrant workers in each of those industries - but looking around, I'd guess that lots of cashiers, lots of farmhands, and maybe a few truckdrivers and gas station attendants are also immigrants. If you had to pay them $30-40/hour, would the apple still cost a dollar?

Only a few of those are labor cost issues, and most of those are not typically migrant labor (truck drivers and grocery employees). That leaves the truck loaders, which is much less work per apple than picking.

Point being, this "$10/lb" is extreme exaggeration.

Now, it's probably true that orchard owners would love the free 3 cents a pound (or 2, or 1). And they're not going to leave money on the table when others can get away with hiring cheaper illegal labor. But the idea that apples have to be that much more expensive is pushing it.