Often it is impossible to agree a price for a project up front because the actual problem or the scope is not clear. In these cases I agree an hourly rate and keep you regularly informed of the billable hours as the project develops.
But, there are a few standard tasks which occur often enough that we can agree a “ready to go” project with a fixed scope, timeline and price.
A great example of this is the project “Get organized” in which in three weeks you will get organized, discover where your time is going, and improve your effectiveness.