You can have a look at a recent thread for a similar car...
http://www.turbobuick.com/forums/threads/5k-mile-86-worth.424059/
Depends on a few things as mentioned:
- Are you the original owner?
 
- Do you have complete service records dating back to the car's delivery?
 
- Have those 3,000 miles been put on in a manner appropriate for the maintenance of the car?
 
- Has it been stored properly?
 
- What other options does it have?
 
- Location of the car.
 
There are other considerations as well.  T-top cars don't generally bring as much as hard top or astro roof cars, for instance.  Still, the car is probably worth somewhere in the low to mid-$30,000's, if everything is in order.  I would ask for more.  You never know.  Low mileage GN's are not hard to come by.  I think one would be hard pressed to meet my expectations for anything north of $25,000.  But then I'm not in the market anyways.  Good luck with the sale of your car.