You won't find Supras for a "good price" anywhere, as ricer demand has far outstripped supply being a rwd toyota, what with the "drifting" craze and all. Otherwise you and I would both have one, I lived next door to one for a year and it seemed to be a great car.
Anyway, the value of the older cars you speak of are mostly dependant on the condition of the vehicle. There is no one dimensional way to price them, like there is a two year old car (outstanding, good, fair, poor isn't enough for these older models). There are services that list wholesale auction transaction prices, but I haven't found a free one. You can also keep an eye on ebay/autotrader for something similar to what you're looking for.