Value depends greatly on what your plans are for the vehicle.
If you are considering purchase for immediate resale in present condition, then you may be willing to pay a little more in exchange for quick profit.
If you plan to put it to daily use, then you must consider your annual maintenance cost on a 25 yr. old vehicle. There are many rubber parts that simply deteriorate with age regardless of mileage or controlled climate, once driven regularly you will find out rather quickly what these parts are.
One should also consider that between the "cash for clunkers" program and the incredibly high price the recyclers are paying for scrap metal, there appears to be a shortage of good used cars. It is shocking how much dealers are asking for any thing with less than 100-k on the odometer. Speaking of odometers have you checked to see if the one in question works(they commonly fail after a few short years and need to be repaired). Accurate mileage on a car that you plan to keep is only really important for mileage calculated repairs such as timing belt, which on a 40.000 mile car means you still have the original 25 yr. old belt due for immediate change based on age alone.
"NADA" is one of the most recognized used car value sources by insurance and finance companies ,so unless you have your car appraised higher than what they say its worth prior to an accident,then this( http://www.nadaguides.com/Classic-Cars/1986/Volvo/240DL/4-Door-Sedan/Values ) is about what you can expect to get out of it when they determine it is totaled because the back bumper fell off, which is another time related repair that will need to take place due to dissimilar metals and electrolysis.
Color is a big deal too,it may not matter to a lot of people but have you tried to sell an ugly car lately, especially one w/o a sunroof and no A/C?
The seat cushions have most likely stared collapsing.
If the dash is not cracked don't take your eyes off of it even for a second,it will split.
All that negativity being taken into consideration and knowing my obsession with volvos,I would would likely consider paying somewhere between $1500-$2500, but would be trying awfully hard to get it for $600.00
Good luck