https://sites.google.com/site/mikefocke2/buyingorsellinga1997-2004boxster is a bit old and surely prices are in 15 year old US dollars but it will give you a start. There is a part 2 and a list of model year differences on the site as well.
Owned 2, loved 'em. Never left stranded. Maintained fanatically. Which is the real secret to buying. Find one that is owned by someone who loved them and spared no expense and who is proud to show you the maintenance records,