I would simply ask your shop what it would cost to make everything work as it should and go from there. It is always hard to put a price on how high is too much. Your personal budget, sentimental value the bike may have, how well the bike currently fits you, your cycling goals, etc all will dictate if it is "worth it" or not.

If your goal is simply to have an efficient commuter that is cost effective and the bike fits you well, fixing it up is probably your best option. The nice thing about 20 year old bikes is parts are usually pretty cheap, the labor can get pricey if there is a lot needed but still well less than a new bike. The other option is to buy any parts needed and fix it yourself or with the help of a co-op. This time of year bike shops are usually slow so you may even be able to talk them into teaching you a bit as they fix your bike or ask if they offer classes on repair. If asking for anything out of the ordinary, a good quality 6 pack goes a very long way in a bike shop!