Project Manager costs are not usually tied to specific work packages but would span the project's lifetime at varying percentages. 
I've seen organizations that have used a flat rate allocation which varies based on the complexity of a project. 
For lower complexity ones, this could be as low as 5%, whereas it could explode for higher complexity projects. 
Alternately, you can look at the life-cycle which the team will follow and allocate a percentage based on the PM's involvement in each phase.