This open-source tech, constructed with bicycle parts, is designed so that it can be constructed for under $100 USD.
3rd Gen Leveraged Freedom Chair - MIT Mobility Labs
Link to Cory Doctorow’s piece, Wheelchair for the developing world: cheap, rugged and easy to maintain, on BoingBoing.
The Core77 Article: Case Study: Leveraged Freedom Chair, by Amos Winter and Jake Child.