I have read that the use of contractors is also a huge driver of cost. In the early and mid 20th century, NYC had a small army of public planners, engineers, draftspersons, and architects on the payroll for public works. As dedicated staff, they were familiar with their specialized area of work and were well integrated into the bureaucracy. After decades of budget slashing, most of those public servants are gone, and now the city must hire contractors who are not well integrated into the city planning system, who must do research and extra planning for jobs they are less familiar with, and who command a far higher hourly rate.