Well if your card is in a very good condition then you might get $6 to $12. I would strongly suggest you to go through a price guide. You'll know how much is your Mark McGwire card worth. The following websites will help you a lot.
www.beckett.com
www.vintagesportscardmemorabilia.com
www.beckett.com
www.vintagesportscardmemorabilia.com
