Well.. they are getting old, and higher in mileage. '97-99 cars will be the cheapest, then 2000-2002 will be a little more $$ and then 2003-2004 the most expensive. Porsche made a host of improvement for 2000 and 2003. Realize that the first ones are now 10 years old and only getting older. I would encourage looking for a 2000 or newer, unless you are turning it into a BoxsterSpec racer
Be sure to budget 10-15% of price for "incidentals" and perhaps overlooked maintenance. Read the board... lots of potential problems, but great fun to drive!