In planning a project there are many factors to consider. What's most important is what's the priority of your client, what their expectations are, and managing the project to that expectation. E.g. if they are flexible on timing, you can take a more relaxed approach to deadlines; however, if you have a fixed deadline, every day late will affect the project and you have to be far more diligent. The key here is to understand what is the driver of the project, which will be based on the client's expectations that you have established as a part of the project planning workshop.
What's important to realize is that whatever you plan upfront is more than likely to change, so you need to be able to adapt and change your plan if and when the need arises. There's no such thing as the perfect project plan. Each project requires its own plan and the only thing you can be sure of is that it's unlikely to work out exactly as you'd planned it.
With pricing a project, for internal projects it's a...