Dont look only at the state, there is a reason why cataphracts are cheaper for some factions. For example, the hellenich cataphracts have such a bad moral. The problem ist just because the roman infantery is so extreme cheap like you already said. A example, the leavy troops of the hellens are as expensive as the roman standard infantery in the polyiban era.

Hetairo are very expensive with 4600 money. To take more than one of them in a 36k game is possible but than you get a crappy infantery and the only chance to win the battle for you is the cavalry. ASM won against a roman player like you can watch with 2 Hetairo but it was extreme close altough he used his cavalry excellent.