Scalping, overcharging for services or products can be an illegal practice.

Overcharging?  

you are getting overcharged, I pay 15% for boots on the ground concierge, to this add 11% spurious lodging tax and 4% miscellaneous extortion fee.

Also, I have experienced lots of buyers being upset because they were overcharged. 

My father earned a living working for a large publishing company whose only job was to fix every shipping bill from shipping companies who always overcharge. 

What are some ongoing items that should be reviewed periodically to make sure you are not being overcharged?

Our contractor on the last house overcharged us big time on his quote so we had to find a new guy who came recommended by a friend.

In my opinion, the more pertinent question is if it is more rational to build a business plan around what is likely to happen, (assuming you take care in choosing your PM):An honest, competent and imperfect, (Read: Human) Property Manager does their best to maximize your ROI while staying true to an ethical and moral standard that is in alignment with yours.Or should you build your business plan around the worst case scenario:A dishonest, incompetent, and imperfect, (Read: Human), Property Manager, not in alignment with your ethical and moral standard, somehow manages to fool you in the interview process and gets away with overcharging you.Both scenarios are possible.

They have to know you will notice overcharging. 

You should also look into a few quotes in your area to make sure your not being overcharged.