Parthian Spearmen are rather cheap and comparably resistant to arrows (from the front). They make good garrison troops.

The basic Iranian combination, which is really cost-efficient, would be Parthian Spearmen, Eastern Axemen, and Persian Archers (or Archer-Spearmen if you want to use them more aggressively).
Other than that, Shipri Tukul and Thorakitai are a great asset against heavy cavalry.

Personally, I would try to use several different ethnically more or less homogenous armies at once. The Seleukids are probably the best faction to pull this off due to their extremely varied recruitment options. You can field a Makedonian army alongside a Persian, Saka, "Roman-style", Galatian, Mesopotamian and Indian army each.