Most natural building systems are cheap when you can get the material locally and readily available ....And you do as much of it as possible yourself.....so if you are prepared to wait and do it slowly, (preferably like on site or near by) and do it bit by bit your self then provided you don't have to pay much for the material then you can build really cheaply. If you don’t want to do it yourself, then eco-friendly buildings can cost as much as conventional buildings.
(Also please see “How much will an Eco Building cost???? )