The workflow in case of software as a service integrates various web services depending on the needs of the user. The same is in the case of the federated ERP (FERP) system, which its functionality is provided from independent providers as web services, which are integrated as a single system against the user enterprises. The composition of FERP web services is carried out by a mediator who is introduced and described as FERP Mall through many previous papers. A FERP Mall aims to cover the needs of small and medium enterprises (SMEs) to ERP functionality as integrated web services that are provided from various providers because the conventional ERP systems are mostly expensive for SMEs because of the related needs like high-end hardware, software, and customization. For the intermediation of FERP Web Services, the FERP Mall (as mediator) will provide the appropriate FERP Workflow (Wf). The relevant questions, in this case, are: How to find the appropriate price of each FERP workflow description as a marketable product? And how the mediator will logically determine the workflow reengineering price also when the user enterprise asks him for business process reengineering. Therefore, this paper aims to derive a mathematical model based on a logical analysis of the complexity of this product (Wf definition) because no like this model has been provided in the literature. In contrast, the previous researches focused on the workflow composition methods and technologies, and few of them provided some considered complexity issues. The targeted model can be considered for determining logical prices of workflow modeling and workflow reengineering in case of web services composition as a separate product.