You can search bikepedia.com to see what year it is and find complete specs and original selling price, FWIW. Looking one ebay or any craigslist or other bikes-for-sale sites to see what this bike has sold for will give you some idea of fair price. At $200, if it's in good shape and it fits you, buy it.

OK. A 3 minute google search.... New 820 is only $350. Older models seem equally low end. It has been in production for many years. Looking at ebay, completed listings.... I would offer $125 and be willing to pay $150 if it's in good shape. If he still has the owners manual, perhaps the owner takes good care of his possessions.