It depends on where on earth you are, but here in Australia, for example, you can buy smartphones for about $10 when on special, or sometimes for free.
[1] Obviously using the solar generated power for something else to save on electricity might be more effective, but where is the fun in that?
> $12.30: 5W 5V Portable Solar Panel Power Bank Phone Charger
I could definitely picture myself as a teenager getting a bunch of shitty old Android devices to make $20 a month, though. My younger cousins will fill out surveys with fake info to make a few bucks here or there.