We have a Classic & Vintage appraisal subforum on this site (linked below), which would be the ideal place to ask.
https://www.bikeforums.net/classic-v...als-inquiries/
But since we’re already here... Speaking in general terms since I can’t see the bike and don’t know where you’re located... Older rigid (non-suspension) mountain bikes generally don’t fetch high prices. Your Cannondale, while it
is a Cannondale, was at the low end of their lineup. I’d guess somewhere around $100 to $120 if it’s in very good condition ...since that’s about what I paid for my daughter’s ‘91 SM400.