Beginning arm cortex Ax hardware development
I've been blogging about the Linux board I've been working on starting here and continuing here. I started thinking I would use a Cortex A8, but eventually settled on the Atmel AT91SAM9G20.

To try to answer your other questions:


You can see the components I used in my schematic, but I don't know of a more general explanation of why each is necessary.
The processor is only about 10-20% of the total cost of parts. Assembly is another $4-40, depending on the quantity built. I suspect the profit margin is 30-50%.
Depending on the package, the OMAP3517 BGA package has either a 1 mm or 0.65 mm ball pitch. Generally, below 0.8 mm pitch, you need to X-ray at least some fraction of the finished boards to check for errors. (Just for the record, the OMAP3517 1 mm packages aren't actually available yet.)


If you have other, more specific questions, I'd be glad to try to answer them.