While you can get phones at these prices, not that they are locked to a provider and subsidized by them. The cheapest 3G+Wifi no-strings-attached phones I could find whenever I looked were always $35-$45, and it's been that way for 3 years now, and they always have mediocre specs (low res screen, low capacity battery, old android).

But they ARE usable for many uses RPi is, and they do have a working 3G and WiFi, and also a touch screen, a speaker and a battery