The 928 is not a cheap car to maintain but if you buy one that has been looked after it is not crazy either. I would guess if you do all of the work your self it would be $500-$1000 a year to maintain.
My car is in great shape but needs some work,
Motor Mounts & Pan Gasket
If I did everything at my mechanic who charges $60 a hour that would be a $3k bill and that is the cash price!!!
I did buy the car cheap ($5500 Canadian) knowing it needed some work but I have put about 12k miles this year and if I did the work I would assume it would not need anything major for awhile.