I also bought the cheap coloured ones on ebay (£2.99 plus £5 postage from China - about $10 all in). They were so cheap that they were worth a punt. They were advertised as OEM and they came in original Porsche boxes but I doubt that they are OEM. However they fit fine and they look genuine so who cares. I see that ebay have removed the listing now but they don't say why.
What I don't understand is why Porsche Tequipment market loads of different part numbers (depending on wheel) for these 75mm centre caps, varying in price from about £130 to £175. It looks to me like, provided the diameter is right, the same caps will fit all wheels. Certainly the ones I bought are not specific to the 19" Cayman wheels I have.