$400-$600 is probably about all I'd spend on a '79 240. Pull the carpet up and make sure the floor isn't rotted out, because that is the biggest problem facing a Volvo (or any car) that old, especially if it's in Massachusetts. If it's under 300K miles, it's probably got some life left in it. It might be worth up to $1000 if it's got low miles (around 200K, give or take) and it's had some recent major work like suspension bushings, steering rack, brakes (which could include master cylinder, lines, rotors, pads, and maybe calipers), fuel pump, etc.
At 104K, the car was either stored or not driven for at least a few years, or it was hardly driven at all (under 5K miles per year is REALLY low). That means it probably drove a lot of short trips, which can be hard on an engine. The B21F engine is bullet proof, so it's not like the mileage is a problem, but if you end up buying it, use a good engine flush (Amsoil makes one) to clean out all of the old oil and deposits.