All unit are relatively expensive to upkeep but the worst offender is the horse archer. For example Turcoman Tribesman cost like 400 upkeep despite carry no armor and weak in melee meanwhile Junior Ghulam Horse Archer which are much stronger armor and can actually melee has 350 upkeep.

Right now i divide my unit to two types.

One that has less 8 armor which you should find with upkeep as cheap as possible because they are too weak.

And the one with 8 armor or more in which case the price didn't really matter because they are a powerful unit that would made up a core of your army.