It is neither overpriced nor underpowered. Assuming the Kontos is used, you'll have the upper hand against every other cavalry and you can also beat spear equipped infantry. The playstyle required to succeed with the cataphract is different to that of other cavalry units so any comparison between them doesn't make much sense such that, for instance, lances are much superior against non-spear equipped infantry and archers so they fulfill somewhat different functions. In short, in my opinion, cataphracts should ideally be used for anti-cavalry and anti-spearmen purposes but they can also be almost as effective against other infantry types as well.