I agree with the DPB Economic Impact Analysis provided for 18 VAC 10-20.  In my 25 years of public service it has been my experience that given similar circumstances professional services contracts generally are more expensive than non professional services contracts. The quality of the work can be controlled by careful attention to specifications, reference checking and proactive contract management.  There is no substitute for cost comparison, and the time consuming professional services contracts will raise the price of photogrammetric services.