The price of a kid's tree house is highly variable depending on its size and the quality of materials used to build it. A cost-effective way to build a tree house is to first buy a set of plans, available from retailers such as TreeHouseSupplies.com, and purchase the supplies separately.
Purchasing a set of tree house plans and supplies still requires hiring construction workers or having enough knowledge to construct the tree house yourself. There are several more expensive options for purchasing custom, fully constructed tree houses, such as BlueForest.com. This company operates through a consultation process whereby they design a tree house for each client's needs and then create a quote reflecting the cost.