The following items can replace the items described in the $4000 system in order the significantly lower the cost.
A pressure treated plywood tank is built by the homeowner.
Two 4 x 8 foot sheets of pressure treated plywood are the sides on the tank. Two 8 FT 4 inch x 4 inch posts yield four 4 inch by 4 inch posts that connect the plywood at the corners. Lag screws hold the tank together.
A 15 ft by 19 foot heavy duty plastic tarp forms the bottom and inside walls of the tank to make it waterproof.
A plastic bulkhead flange is used to create a 3 inch output port for a 3 inch suction hose which delivers water from the tank to the pump.
3/4 inch plywood is recommended. Pressure treated costs more, but is best choice if homeowner can afford it.
When the homeowner starts the pump, the fire has, hopefully, not yet arrived. The 2-port valve is closed and no water is sprayed around the house. If no water flows through the pump, the pump can overheat.
The garden hose guarantees a small flow through the pump and prevents overheating. The water exits the garden hose into the tank. When the heat sensors detect the fire, the 2-port opens and the house is sprayed.