The answer to 'average cost of shut-off valve' is that the cost can vary widely depending on several factors. Typically, a basic shut-off valve for a home plumbing system can range anywhere from $20 to $150 or more, depending on the type, size, and materials used.
The cost of a shut-off valve is influenced by factors such as the valve's material (e.g., brass, stainless steel, or plastic), the size of the valve (1/2-inch to 1-inch are common sizes), whether it's a ball valve or gate valve, and the complexity of the installation. Valves in hard-to-reach locations or that require additional plumbing work will also be more expensive. It's best to get local quotes from licensed plumbers to get an accurate estimate for your specific needs.