I own a '01 S, second Boxster, 56K miles. Over 4 years of ownership, it has never left me stranded and has cost $15 a month in unscheduled maintenance. You can do a lot of the maintenance yourself...and I'm no mechanic. There are lots of do it yourself guides to the typical things you do. No way does a oil change cost $100...not of you shop the parts right.
Can it be more expensive than a normal car...you bet. Blow an engine and you can see some serious costs. But 3 weeks ago I had to change an Oxygen sensor in my Honda CRV and my Porsche on the same day. The Porsche part (bought OEM and mail order) was well less than half the Honda part bought the same way. And the labor was the same at my local tire shop...no need to go to a Porsche specialist for something as simple as this.
And I value my well cared for car with good but not the best options set at $16.5k in today's market so you can do lots better than that $20k and have some cash left over for any needed repairs. Do do a PPI.
For much more on this...follow the link
http://mike.focke.googlepages.com/mikesporscheboxsterwebpages