Well, for starters, a car is only worth what someone is willing to pay.
That being said, without seeing pics of the car and knowing more details about it, it's hard to give a fair amount of worth. I just picked up a 90 SC with 5spd manual and the car is one sweet ride. The previous owner did alot of work on the car and made some good, SENSIBLE upgrades prior to my purchase. I paid $1100 for my car and I feel I got FAR more than a good deal with my SC. I absolutely love the car and am more than happy with it. Of course, what one person thinks a car is worth vs. what YOU think it's worth is also another issue. This car may appeal more to you than it does someone else, hence a varying degree of what the car is worth.
A few things to look for: head gaskets. Have they been replaced or not? How many miles on the car, has the exhaust been upgraded? Check underneath the car for frame rust and floorboard rust as well. Don't be afraid to pick the car apart. Remember, the more you can find wrong with the car, the more advantage you have when it comes time to talk price. Don't insult the owner, just point out what you'll need to do to the car should you purchase it.
One final word of advice. It's very easy to get excited over the potential purchase of a car like this. I know I did.
But, don't let your heart dictate your purchase of the car. If it's not the car you are really looking for and feel like you're settling for it, maybe you should pass. Also, if the car needs more work than you're really comfortable with or unsure of, don't let your heart talk you into buying the car thinking you can get everything fixed to your satisfaction. More times than not, you end up with more headaches than you bargained for. Case in point. There is an 89 SC for sale here in this area. It's been for sale almost a year now. Anyways, the price started at $3000 I think it was. I went to look at the car last summer. Now, the owner has done alot of work to the car. Head gaskets, supercharger rebuilt, all new suspension bushings and so forth...tons of work actually. BUT, when I looked over the body of the car, I noticed the front fascia was broken and torn underneath the car. The passenger driving light was also broken and twisted in a bad way. When I looked at the back bumper, I found tears along the bottom and the paint was cracked and it was obvious it had been repainted previously and a crappy job of paint prep was done. The front bumper cover was also in the same condition. With the suspension parts that were replaced and the damage done to the bumper covers, there was enough evidence consistent for me to believe the car had been in an accident.
Well, here it is almost a year later and this car is still for sale and the price is down to $1700. I looked at and drove the car again a couple weeks ago. But, in my mind, there was a nagging sense something with the car just was not up to par. It was a choice between that car and the SC I purchased this past week. Needless to say, I am more than happy with the choice I made. But, what I'm trying to say is, it would have been very easy for me to buy the 1st SC I looked at. But, instead of following my heart, I followed my senses and held off for the right car. It was definitely worth the wait!
Sorry for the long replay, but I just don't want to see you go look at the car with your mind already made up that this is the car for you regardless. be objective and be honest with yourself about the actual condition of the car. You'll be glad you did in the end.