@Joe Kim I do not mean to offend you in any way, but rather open you up to a new way of thinking. As a real estate investor you have a lot of the control. I do not know the PM's situation nor yours, but your time is extremely valuable as is his. With that said you have to ability to acquire more rental properties and feed the system you have created with your PM. Your "empire" can be much larger than his and you have the choice of who maintains it. The only capability he has is to maintain the system. While 6%-10% may (and possibly is) a lot for a single property, spread that over more rental properties and watch as this will benefit you both over the long run. Use future business as a negotiation tactic to lower your costs per unit. If you bring more volume, then he can lower his prices per unit.
Like @Ryan Murdock said, you can always look for someone else to learn the system already put in place and begin to phase your current PM out. Usually this could be a newer, more energetic PM who in return for all the business will provide lower rates. Always be talking and vetting new prospective vendors for your properties. I say if you feel they are making to much per property, then purchase more, give them more work and receive the rewards of all the more rental properties you have. Best of luck!