a non-profit organization is wanting to create outdoor pathways that are accessible to people with disabilities. When taking a low budget into account, what are the best materials to use? The goal is something that is accessible, durable, low maintenance and affordable. cement/asphalt/crushed limestone???

Asphalt and concrete required some expertise to install, especially when attempting to conform to ADA Accessibilty Guidelines (ADAAG). Those would be the most expensive. Crushed limestone, crusher run, stone dust, and the like can be constructed as a paving surface without the 'expertise'. But understand that meeting the black and white guidelines (ADAAG) can be excrutiatingly difficult, especially if the ADA folks having jurisdiction in your area are sticklers for detail and absolute conformance to the law. There is no give or take on these issues. The law is black and white, and ADA inspectors have no latitude to say 'close enough'.

Know what you're getting into if you, or the folks you refer to, 'volunteer' to do a good turn here. It may be a larger effort than you think.