The first question is whether the bike fits you well. If it doesn't fit you, don't buy it for any price. Bikes from that era vary from bikes that were used for a few months, hung up in the garage, and are now still in "like new" condition, to bikes where every bearing and cog is worn out.
Take the bike to a good bike shop and have them give you a written estimate for the cost of tuning the bike up, truing the wheels, and the cost of fixing any problems that they spot. Subtract that estimate from $150 or $175 and you have the "bottom line" value of the bike.