[paulk]

Even if your friend''s sail is pretty blown out, it''s likely in better shape than the 20-year old one you have. What your friend''s sail is worth to YOU is the question. If it is in reasonably good shape, it could last you 15 more years. If you plan any racing, it is again likely better than what you have, but perhaps not worth spending too much on, since you would need to replace it with a new sail sooner (1-2 seasons), rather than later (the 15 years mentioned earlier).
Bacon''s Used Sails website should give you an idea of what used sails cost.