A solenoid valve generally costs from €24 to €355. The price mainly depends on the solenoid valve diameter

How to calculate the dimensions for a solenoid valve?
To determine the optimal size for your solenoid valve, you need to know the flow rate, which allows you to determine the flow factor (kV). The flow factor is a theoretical value that indicates the volume of water at room temperature that flows through the solenoid valve with a pressure drop of 1 bar for one minute. It can be indicated in liters per minute (l/min) or cubic meters per hour (m3/h).

You will also need to know the inlet pressure and determine the outlet pressure, which will allow you to define the pressure drop of the solenoid valve. For a pilot-operated solenoid valve, the pressure drop must be at least equal to the minimum differential pressure necessary for the system to operate properly.

Then you will need to know the diameter of the inlet and outlet pipes (indicated in inches or millimeters) and the type of connection (threaded or flange mounted).

The choice of the number of ports on a solenoid valve (2, 3, 4, or 5) will depend on the intended use:

A 2-way solenoid valve is mainly used to supply a circuit directly, for example, to fill a tank.
A 3-way solenoid valve is mainly used to regulate a circuit. 3-way solenoid valves are generally used with two inlet lines and one outlet line to ensure the mixing of two fluids with constant flow. They can also be used with one inlet pipe and two outlet pipes to regulate the flow at one of the outlets by diverting the fluid flow to the second outlet.

4-way solenoid valves are equipped with two inputs and two outputs and are used, for example, for reversing cycles for a heat pump.
Lastly, you must pay attention to the supply voltage of the solenoid valve.

Please note, some pneumatic directional control valves or hydraulic directional control valves may be similar to solenoid valves but their operating principle is different.

They are mainly found on pneumatic or hydraulic circuits to control actuators.