I would argue that the Native Levy Phalanx is awesome if you are a faction that can possess them, they are a cheap 240 man phalanx unit with AP axes as their secondary weapon. Really this would be my overall choice in the "bang for the Buck" category, although they are not perfect (low armor and middling morale) they are cheap, numerous and have a very wide recruitment area. Their AP secondary weapon makes them ideal for holding walls, and they are much better than the average phalanx unit at taking walls as well.

Peltasts are cheap, have a wide recruitment area, and they can play a key role in battle in a number of ways. First, they are obviously skirmishers, but more importantly they have just enough armor and melee power with their shortsword to engage heavy units, and they can tear things up in flanking maneuvers. Also, they can be used as light assault troops in sieges. I used to hate these guys, but they are really useful, widely available, and cheap. Probably not what you are looking for, but worth consideration since they are available to every faction.

There are probably a couple of dozen good expensive units, but of those I think that Thoraktai are a solid choice for somewhat expensive armored killers. Also almost any of the Roman line units are vicious.

Ethiopian Spearmen are a great cheap unit in the Red Sea/Arabia area. I really like these guys as they are cheap, quick, and effective.

Northern Gallic Swordsmen - pretty much a standard Celtic line unit that is available as a merc throughout Europe. They make good effective mercenaries, and solid cheap troops for the Celtic factions.

The ultimate bang for the buck unit - Hellenic slingers Well, if you want to kill people, and horses for that matter, and you can't afford badasses then this is the answer. I don't like it, and nobody else does either, but everyone has spammed slingers before. I try to keep legitimately 4 units in my army, but if I am broke i might end up using garrison forces that swell the slinger total towards 1/2 stack. Ugly, but effective, Armor piercing, widely available, and the cheapest unit in the game (400 mnai for the hellenic version).

Persian Archer-Spearmen are noteworthy because they are flexible and dirt cheap. I think they are priced around 600 mnai, so they are disposable garrison troops, but you can take them on a campaign. Since they are spearmen, they can actually hold up to a cavalry charge - also when they run out of arrows they make a great reserve force to throw into melee. They are not as armored as the Peltasts, but they are otherwise similarly useful, although their specialty is defensive rather than offensive (except for all of the offensive arrows). As Baktria or Pahlava, or Saba you will probably rely on these guys to hold your cities against numerous seiges.

That's my list of "Bang for the buck" units - no expensive supermen (elephants can kill thousands, but they are very expensive) and no Romani units, since I don't know their roster, but just a bunch of cheap reliable killers that will give you your money's worth, especially since you aren't paying much for them.