Assuming the wheels are OK and that there are no dings in the frame I`d go for $350 assuming I`d have to clean it up, replace all cables and change the tires, tubes and bar tape.
Because of the age of the bike I`d expect to have to do a complete teardown and relube everything. Thing is - equipped with Schwalbes, a Brooks sadle, new bar tape and racks front and back - that would be a nice ride!