Depends on parts. A rebuilt OEM alternator form Pelican Parts is listed at $1,371.75. Reman Bosch is listed at $319.00. I remember a quote from the dealer several years ago was north of $1,700.00 for new OEM.
Depending on your DIY skills, it is a reasonably straight forward repair;
https://www.pelicanparts.com/techarticles/Boxster_Tech/82-ELEC-Alternator/82-ELEC-Alternator.htm
The voltage regulator is an easy/cheaper repair also.