First, a client is always right and I think ball valves are more expensive but also safe and robust, a butterfly...might be cheaper but can also impede flow ie pressure drop so probably depends on what is in the pipe(mechanical abrasion, jamming).

It may be driven by the pipe spec which calls for a ball valve.
Often I see a plug valve as a substitute, I think they are less expensive to manufacture in larger sizes but may require a larger actuator.

Thanks for all your replies. Actually these valves are used for clean effluent water and it is not from the pipe spec but only in the Instrumentation spec.

I agree with UniCO2,that there will be significant cost variation between ball and butterfly valves but is it that the butterfly valves will be less reliable than ball valves in terms of failure of components?

The max upstream pressure of the subject valves is 4bar only.

You can see the cycles to failure on the spec sheet for the valve. I would think you can buy a very expensive butterfly that is better than a cheap ball valve and vice versa. Could be, when they do fail how they actually fail(open or closed). Lastly - the pressure drop with a different valve (butterfly) might affect all the other calculations for the system.