I don't know what the dealer charges but here's an OEM from Ebay for $61. Same seller that sells the double din kits.
PORSCHE 986 BOXSTER RADIO SUPPORT SHELF (CUBBY BIN) | eBay
Nice install BTW. Hope to do that to my Boxster eventually.
Decided not to get the cheap Chinese double din.
Did you consider the Pioneer unit or is Kenwood the way to go?