This is very difficult to answer in traditional accounting terms because the real value is in the quality of the end results and the avoidance of unnecessary delays and costs. In short, stakeholder and customer satisfaction. It's like taking out insurance, no one argues about taking out the right amount of insurance. No one should argue about doing the right amount of project management.
Remember that Murphy, that good old proponent of humorous laws, has said "A poorly run project will take three times as long and cost three times as much as a well run project..." However, he also added "compared to a well run project which only takes twice as long and costs twice as much." For our financial accounting friends, that's a 50% saving right there!