When you hire a consultant, you only know if you’re paying a fair fee if you have some idea of what the project is worth to you. Unfortunately, you overestimate the contribution of your project to whatever your larger goal is and therefore you risk overestimating the value of your project.
By the way, this same principal applies to the projects and initiatives you do in-house: you overestimate their value. Why? Let’s go back to 1979 for a moment, when Michael Ross and Fiore Sicoly at the University of Waterloo, Canada wrote about egocentricity biases. Really, you need to know about these.