Larger WordPress solutions take more time in all aspects: carefully defining requirements and development milestones, iterating through different phases, ensuring that the overall product quality stays high at all times.
Working with a dedicated employee or a freelancer usually means that a single person is responsible for the management and technical aspect of the WordPress development process. Changing hats all days and making sure that the project stays in tact is extremely complicated, and a few are capable of managing the balance properly.
A WordPress project manager is responsible for the website project planning and communicating the requirements between the customer and the development team. Experienced website project managers have technical background which is vital for translating the requirements to milestones, and constantly communicating the process from the development department back to the client.
The communication and project management overhead could be 10-20% on top of the development work in most cases. However this is a legitimate expense for the successful completion of a WordPress project.
At DevriX we assign a project manager to each project which usually includes 10% additional costs to the final project. Check out our work process and website development model.