First off it's a Dreamcast...so not very much. Plus someone buying it would somehow have to go and find the second disc which could prove difficult. So i would say you may have a hard time finding any buyer who isn't just looking for the disc you have, like to replace one that is damaged maybe.
To answer the question though i have seen the full game listed on plenty of sites, and prices range from around $30 at the low end, to around $60 on the higher end. But that is both discs. So cut the low end in half and i would say $15-$20 at very most.