Dave,
Great work.
You might want to also contact The Wheelchair Foundation to see what their thoughts might be. Looks like they pay $150/chair.
http://www.wheelchairfoundation.org/
Thinking about where these chairs are used, where would a person in a third world country find air to inflate their tires? Are the chairs breaking because the wrong chair is given to a heavy person? (alum vs. steel) Use bigger axles for heavier people in worse conditions?
Wheelchairs has been a big charity item for many Rotary Clubs, and there is an annual November distribution in Mazatlan.