Perhaps affordable home 3D printing will come from paper pulp. Feed your 3D printer from your paper shredder with a little glue and some inkjet colour cartridges and you might be able to build a really cheap-to-run 3D colour printer. It would be like automated paper mache, so not actually strong or detailed, but good enough for simple decorative models and disposable toys.
The advantages would be that the machine can run on paper waste, common household glue and ordinary inkjet cartridges, all readily available and cheap. That's assuming the whole idea works, of course.
Mokalus of Borg
PS - And is worthwhile.
PPS - It's probably a bit too narrow a niche to be useful.