Must be different on your side of the water, manufacturer I work for there official dealers cant charge any more then the recomended retail price for the part.

Parts price should be the same if you go to the OEM parts counter or any one of the official dealers.

10-20 % is cheap...I know of a couple companies that go as high as 35%.......

I think you will find that is pretty typical. A lot has to do with how much competition there is, the type of customer and type of part. I know over 20 years ago this was not the norm especially with large customers.
The dealership I was at tried to sell at 10 over and got a phone call from the sales manager at the OEM. Also back then customers would call other dealerships to see if you were selling at list. I am guessing that this does not happen so much anymore due to dealer consolidation covering multiple locations and states. Back then we were told that we could not refuse to quote someone out of our territory we just could not sell to them.