For an individual, RPi is cheaper if your time is worth anything at all.
On the other hand, if enough devices of the same type become available for this purpose it might make sense for a business to do this at scale.
On the other hand, if enough devices of the same type become available for this purpose it might make sense for a business to do this at scale.
The only reason that's true is because groups have catered to the device and have ready-to-go instructions. Were the same effort put into making Samsung S2/S3/S4 etc phones easily converted, I'm not sure the time difference would be all that much at all.
This could then be used as a basis for phone recycling programs: manufacturers could run the collection (and e.g. hand out gift cards for its own new products to encourage the users to turn them in while also profiting from it), and then sell the collected phones in bulk to datacenters.