If you post the price, and its better than the competition, you will generate sales for maranello BMW. (Which seems to be what you are trying to do.)
If you dont post the price, even if someone calls/pm's/goes themselves to the dealer and the price is higher, they'll likely buy the part elsewhere, wasting everyone's time and being of no benefit to anyone involved.
These are part sales, there's no ambiguity as there is in labor as to how much work something is. Prices (i'd imagine) are fixed, so why not post them?