If you're on a tight budget and want to just keep this one going then open it up and see what's inside. It's likely that the springs and insides are a bit rusty on a bike that old. Clean it all up and grease EVERYTHING with a water resistant marine grade grease and put it back together. Likely the squeak will be gone and it'll be as good as it ever was for casual riding for a few more years. The only cost will be your time and about 50 cents worth of grease.