I think it's too broad of a question. There are some games that will always be cheap, some that will fluctuate, and some that will always sell for obscene amounts. Original print run, platform, version differences, all this stuff can have an impact, even if a re-release is subjectively "better" than the original.
I think the only area where we'll see a more strict dividing line is with hardware breaking down over time, and limiting the number of reliable units around, but we're a long way off from that.