13K is right around the appraised value of an original well cared for stock, lo mile 87 GN . My GN with T-tops was appraised (and insured for) 14.5K. I turned down a couple 10K offer on mine and wouldnt sell it a penny under 12K . Racers dont want the T-topped cars so they go for a little less than the hard tops. If its clean and tight and not messed up Ive seen them sell anywhere from 6-18 K . It all depends on the condition and urgency of the seller. The Rear main oil seal leak is very common on these cars, and the t-top gaskets if original would be shot. The power antennas are a common thing and a cheap fix with a nice black stick. If well cared for there would be little or no rust and only usual age related issues. The original bumper fillers tend to fall apart by now and lots of aftermarket replacements available. The window sweeps are probably hard and shot unless they been replaced. Also the headliner may be unglued and loose and the foam dry rotted if the car saw any length of sun through the years. All this stuff is relatively easy inexpensive to fix but makes for barter points. It all depends what you want and how long you want to hunt for a deal. If its clean and presents well, good tires, exhaust is solid and it Drives and shifts well with no issues, In my book 10K would be a fair price, 12K would be around avg for cars that are a bit better condition and 14+ for those extremely well cared for, needs nothing cars. My 87 is original and i like it that way. i have had it for over 20 years and when the weather is good I drive it a lot , commute to work, and hit the cruize nights and keep on top of all the normal wear and tear and age related stuff.