what is the criteria to calculate the price and value of a project management work? how many percent of total project price belongs to project managing? (for a low technology project such as cathodic protection o a pipeline.).thanks for your answers.

That depends on the scope of the work and who does the work. Often the more upfront work that you do with experienced managers, the cost of the project is lower. So the percent of the project may change depending on your criteria. The important thing to look at is the total cost of the project.