With Brunswicks - Mercury policy of non-disclosure of company information and inner workings,any employee who did would probably loose his job for chiming in here!
Mercury is just a normal big US stock Corporation, they are trying to make the maximum profit on anything they think they can control thru part exclusivity for the benefit of their stockholders. Lots of similar big industries do the same thing with replacement parts sales. Buy yourself a new BMW and the engine probably is being sold in the car at about $10K. Buy that same BMW engine from parts and I think it would cost $50K over the counter. Same situation. This is also one of the ways they use parts resale profit to offset the other costs associated with their marine engines such as extended warranty, racing sponsorship and such.
Other smaller marine engine manufacturers like Volvo Penta, Crusader, Pleasurecraft and aftermarket builders like Raylar, Teague and Ilmor take a more reasonable profit approach on replacement parts sales because we are not feeding such a large Elephant!
I am not trying to justify Mercury's pricing on replacement parts here, its just important you understand why they do what they do.

Best Regards,
Ray @ Raylar