personally - cheaper prices usually indicate younger fish or stunted fish...whenever you buy Discus and buying from breeders that you can't physically go pick from be precise about what you want - small eye to body ratio, full fins (deformed fins are not favourable), red eyes are favourable, no peppering either (if your fish colour is prone to it).
I have brought expensive ones and cheap ones but the cheap ones I tend to flick later on because they just don't end up looking as good as the ones brought from breeders/importers
The price usually indicates what strain of Discus it is...common strains are cheaper etc