I still am trying to figure out why are non-nomad horses and their riders more expensive than nomad horses to maintain (Because there is no such thing as large contingents of settled Horse Archers, except for the said nobles). And do not be fooled by the conotation "Settled = Farming". Many settled populations practiced both farming and hunting (When able), not only in Persia, but basically everywhere.

Nomads first of all bred horses like settled people breed sheep. They used them for skins, for milk even meat. Since their lands are only for grazing they could raise many horses. Quantity reduced prices.
Second nomad horses were breeds that ate grass and were perfectly fine. The bigger horses of settled civilisations were domesticated breeds that won't stay in top condition only with grass, the also need hay and other kinds of food which adds to the expenses.