Percentage of Budget that should be devoted to pla

In a lot of the places that I have worked in the past couple of years (mainly software development environment) I have found that I am constantly arguing with project stakeholders about the value of project planning. i.e. it is either too hard to do, not enough time is spend on it and too much time is spend on it etc
So I am interested in finding out whether anyone has heard of any published information that refers to the percentage of the project budget adn time that should be devoted to project scheduling.
If anyone also has an opinion on this subject I would also be interested in hearing it.