Not only the cost of the individual valve but the total cost of the installed system. When all costs are considered, including freight, installation and service life, a metal system will in most cases be more expensive. The differences can be significant. Exotic metal alloy systems can cost up to 13 times that of a plastic system . Even carbon steel can be almost twice the cost of PVC - depending on the size and complexity of the piping system involved.
Flow rates are another area where plastic pipe has an advantage.
The interior of plastic pipe is smooth and clean and it will stay that way year after year. Metal pipe can rust , corrode and scale - resulting in reduced flow rates and higher pressure drops over time. Often, users are not aware of the engineering advances that have been made with regard to plastic piping materials over the last several years. Users of metal pipe, valves, and fittings are often concerned with what they believe to be the mechanical strength limitations of plastics. And plastic piping systems are now commonly available that provide adequate tensile strength or operation up to 200F. Plastic pipe is also available that maintains its pressure bearing capabilities for over 50 years.