The Certificate of Authenticity, is, as Jim suggests, about the only way to precisely identify the original cost. You can order one through the PCNA web site.
If you go to
http://www.rahul.net/dennisp/boxster/boxster-1999-options-list.shtml you will find a list of option prices for the '99. Then by using the option code sticker which should be on the underside of the front luggage compartment lid you can attempt to build your own approximation of the original list price. This method is almost guaranteed to be somewhat inaccurate as you will probably find some "mystery" options on your sticker that are not on the list.
Unless your car is either a stripper or a highly optioned car, the MSRP was probably about $50,000 + or -.