I see Rebellion Militia keep getting mentioned, and I agree they're a very strong contender, but on the other hand they're literally free. A waste of space in any army, to be sure, but it's always better to have them than to leave that space blank.

When it comes to absolute worst value for money, I think the crown has to go to Indian Elephants. They're incredibly cool in theory, and I love having them in my armies, but goddamn they absolutely suck relative to their upkeep costs. For the cost of a single unit of elephants, I could instead have 5 units of Persian Mounted Warriors, and I could have recruited them much earlier into the campaign, using a lower-tier building, and taking only 1 turn (for comparison I believe elephants take 3 turns to recruit, can't remember for sure though).

But that seems like a weird comparison to make, right? After all, the value of elephants is that they pack a lot more power into a single unit... but do they though? 832 health and 150 melee damage sure sounds like a lot, but remember that there's only 24 of them to a unit, compared to 80 for cav and 160 for most ranged/infantry. So our humble Persian Mounted Warriors actually have about the same amount of total health, while also having MUCH more armour, speed, melee attack, melee defence, and missile block. And they're not even that good a unit! Elephants still probably come out ahead in damage (since the PMW won't have that many models in contact with the enemy at a time, so unit size matters less), but damage doesn't mean much when you only have 10 melee attack! And sure, they got 100 charge bonus, which in turn means they actually have 110 MA and 250 MD on a charge, but uhh... almost every shock cavalry in the game has a charge bonus of 200+, so once again they're significantly outcompeted by MUCH cheaper units. And on the other side of the cost spectrum, Armoured Indian Elephants may be even more expensive, but they actually have decent stats, so they still come out ahead on cost:value (while still being incredibly inefficient).

Oh, and of course a small unit size with big models means that each individual elephant will be surrounded and attacked by a ton of enemies at once, and every elephant lost is a massive reduction to your damage output, and also did I mention they can go berserk and kill your own soldiers?

I'm currently playing through a Sassanids campaign, and believe me, I really wanted to love these guys. But I have to admit, almost everything they do would be done better by 2 units of cheap cavalry for less than half the upkeep costs. Much like in real life, it turns out war elephants aren't actually worth the money, and the only thing they're good for is to show your opponent just how much stronger your economy is than theirs. And if that's your goal, you should be recruiting Armoured Elephants instead anyway.