The cost of a bush plane can vary widely depending on a number of factors such as your choice of engine and other components, as well as whether you intend to buy it factory-built or as a kit.
Typical prices average around $80,000 to $100,000 from most kit manufacturers, however, it really depends on the options you choose and what you require from a bush plane. This price would just be for the actual components and kit alone. Around 1000 or so hours of amateur construction would then be required to turn a pile of parts and components into an actual flight-capable bush plane.
Some manufacturers do offer fairly well kitted out factory-built options if you don’t have 1000 hours to spend carefully piecing together your new bush plane, however, this does come at a cost and can add a serious premium on top of what a kit would cost.