I bought my 03 Boxster S in 2008 with 14k miles on it. The only maintenance it has needed was a recommended brake flush (when I first bought it) and an oil change every year. I've had no unscheduled maintenance issues with the car and it has been 100% reliable. The only unusual expense was replacing the IMS bearing, which I did as an elective preventative maintenance item (about $2300 including new clutch and at the time, the original bearing and clutch were in fine shape).
So now with 31k miles on the car, it has never actually needed anything, so it's been about the same cost to maintain as my Lexus SUV or Toyota Tacoma.