I am toying with the idea of upgrading a new, non-AMG R231 SL with the AMG Ceramic Composite brakes. The parts are HORRIBLY expensive, like $15K to $18K, but I am trying to get the dealer to cut me a special deal, since I am buying the
new car from them. Just so I know how far I can push, does anyone know what the dealer pays for items such as this? Does he get them for 50% of retail? More? Less? Maybe there is an MB service or parts employee who might not
mind sharing some inside knowledge here? Thanks in advance!

If you assume 100% markup, you will not be far off. Not possible to answer the question with certainty unless an insider, or former insider provides information.

i have an MBUSA parts list that I grabbed but it is outdated. here is a random screen shot
and it may give you an inkling. no doubt the % is higher now. dealer price, followed by retail.
multiply by .01