How Much Is A 1989 Fleer Baseball Card Set Worth?

According to the 2009 Beckett Baseball Card Price Guide, a 1989 Fleer set is worth between 6 and 15 dollars depending on the quality of the cards.
Beckett Baseball Price guides cover most of the value of sets and individual cards. It is available at most book stores at a cost of $29.95. (that was the cost of the 2009 edition)
Strangely, individual star cards are worth more when their value is added together than the value of the entire set.

Inherited old baseball cards.  Still in boxes and wrappers.  Where should I go to sell them or find out what they are worth?

You're not going to like the answer, but the complete (boxed) set is only worth around $5.00 or so. Fleer cards from the late 80's were so mass produced that there is really no value to them. The same goes for Donruss and Topps.