When I want to know what something is worth, I like to go to ebay. And then I look to see what similar items have sold for. It does me no good, for example, to see that someone has an asking price of $200 on a card when actual cards are selling for $10. They're listing it with no actual intention of selling them. I've seen cards listed for over $1000 when actual selling prices are around $20. I can't even tell by looking for a Griffey rookie card under sold items because a number of different rookie cards were produced. The best thing I can suggest is to look at items that have sold and find the specific Griffey card you have. I performed a search for "Griffey Rookie Card" and specified sold items. 4700 sold listings were found. Just on the first page I found cards that sold from a low of 99c to a high of $225.99. So they vary widely. I am attaching a link to my search results. Find ones exactly like yours that have sold. That should give you a pretty good idea.
http://www.ebay.com/csc/i.html?_from=R40&_sacat=0&_nkw=griffey+rookie+card&LH_Complete=1&LH_Sold=1&rt=nc