Check autotrader and the like - see what people are paying. Better still, pm chowetime, she has just bought a skyline after looking at many cars, most of which she was dissapointed with - perhaps she can give you a feel for what is on the market and how much it goes for.
Remember, its pretty hard to say what a car is worth, until you have seen it yourself - there is alot of difference between a well modded, well maintained car in excellent condition (when I bought my 94, everyone who saw it and didnt know a thing about skyines thought it was a new car - it was that neat) and a car that hasnt been looked after. As Ken said, there are dogs out there
tdawg, my 94 with mild mods, in near immaculate condition and 80k/km sold for more than your 97 series 2 - either
you underpriced your car,
some people put more value on mods than you do, or
your car was in average condition even with the low kms