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
Not really. The prices have dropped tremendously. I recently picked up the Alcatel C1 for just $9 at the local frys. Sure it is under powered compared to the flagship phones, but it is really capable enough for tinkering, it also has a touchscreen and comes installed with Andriod KitKat 4.2, which could also be rooted. [1]
The low end Lumia phones always go on sale and has amazing specs for what you pay. For example Lumia 635 (on sale $19-$49) has IPS display, 5mp camera, Quad core 1.2GHz and a good amount of sensors. [2] However the gripe with these is these are not as hackable as Andriod, so good for backup phone but not so good for DIY projects.
[1] http://www.cnet.com/products/alcatel-c1/ [2] http://www.gsmarena.com/nokia_lumia_635-6254.php