And to balance it out how some units are ridciously cost effective then the following units must always be upgraded (once):

Alpine phalanx, dacian falxmen (maybe 2 chevrons), polybian principes, polybian triarii, Thracian peltasts, Levy phalangists, Rhaetix axemen, Bosporan archers, Imperial eastern archer auxilia, maybe cretans......

For if these units get more expensive then the player has less money to make other units as powerful.