The main challenge of exposing IEC61499 or IEC61131-3 Function Blocks as a service remains in adopting service-oriented concepts in function block programming. Implementing an IEC61499 or IEC61131-3 Function Block that is being accessed via service-oriented protocols is straight forward. The main challenge remains in modeling a Function Block as a service. Adopting service-oriented concepts like Service Oriented Architecture or Microservice Architecture requires tackling challenges like service granularity, (de)composition, etc. For instance, too coarse-grained services could lead to significant drawbacks, while too fine-grained services could increase the system's overall complexity, introducing semantic tight coupling and bringing about communication overhead. Therefore, understanding whether a service (de)composition is adding any value could help us to identify the best service granularity. Finding the best service granularity means knowing how many Function Blocks could be exposed into one service. This could undeniable lead to improvement in resource consumption especially in constraint environments. In this paper we design a cost analysis function for calculating the overhead of service decomposition. This work will help to answer one of the most important aspects of the service-oriented approach, called service granularity in the scope of Function Block as a Service.