I have just replaced all my tyres. I have perilli pzero tyres all the way round. 205//17/zr N3 for the front and 255//17/zr N3 for the back. Porsche reccomened running the same tyres throughout as this is why i replaced my front tyres which were still ok. I replaced them whole tyres with the same treads going the same way. It cost me £740 thats around $1320 and this was cheap as i know the guy who works there. but at the end of the day its the only thing between the car and the almighty. Quite expensive but it runs fantastic, sitting at around 130 mph is like an air pocket, its great.
remember the better the tyre the better drive you will have and less road noise. cheap tyres and your asking for trouble.