Apple's CarPlay
 

Mercedes has confirmed they will have an aftermarket retrofit solution to bring Carplay into older Mercedes vehicles.
http://9to5mac.com/2014/03/06/mercedes-benz-working-on-aftermarket-carplay-installations-for-older-vehicles/

With Porsche's PCM system already having all the hardware (USB interface, touch screen, etc), it'd be much easier for them to come up with something. Most likely it will be a re-write of the PCM firmware, but that's got to be cheaper than swapping out the whole system.

So what do you guys think? Will it ever happen? How much would you pay for something like this?

Never happen. There are things they could add from later revisions (e.g. album art) to the older ones, but don't.

On the off chance they do offer an upgrade, it might make more financial sense to just get a new CTTS...

Agreed...