Even if Magus is cost efficient, AP junglers are barely a thing. They still have mana problems for a large portion of the game when they are supposed to be helping out other lanes. Blue buff helps, but isn't constant. It's difficult to make an item for them though. AP junglers are tricky in that they are either Elise and Evelynn, who don't make good use out of high AP and don't scale well past the early game. Or they have pretty decent power throughout the game (some need lvl 6 to be effective) Fiddles, Diana, or Nidalee. This is in contrast to Warrior, Cinderhulk, or even Devourer users. Warrior using champions typically strive in the early game but fall off hard later in the game. Rengar and Kha'Zix are the two champions who build much more offensively than other AD junglers due to them being assassins (oh forgot Shaco), but you will see most Warriors going for tankier builds after the early game so they can remain relevant. The Warrior enchant optimizes their ability to dominate early, but is an item that reflects it's user in that it's power is ONLY for the early game. Cinderhulk is kind of the opposite. It is for those junglers who cannot put up heavy pressure early, but trade that in order to have a much stronger late game. Devourer gives a stacking mechanic that rewards heavy farming junglers so they gain power and have strong on hit effects with their AS; it caters to
and other AS/AA based champs who can become incredibly powerful lategame due to the enhance power of their autos (thought I think it's underpowered).
Magus seems to be the enchant that really doesn't have an identity. It's just a ball of stats that generally says "This item helps the power of your abilities because AP and CDR", but it doesn't give an idea of how to use the item best. It's really just the item you have to take on AP junglers because there isn't another item to choose from that will benefit you. It gives okay AP and good CDR but you still need to buy a mana/mana restore item to stay in the jungle or stay in a fight, and you cannot be a full carry when you have to keep building (60-80AP) mid tier AP items so it really limits your ability to scale into the late game, Your strongest point should be your midgame, but since you will just have your jungle item and a RoA, you won't be outputting a lot of damage as compared to champions who will have more due to increased income and only 1 "starting" item. (I will note that this is only really the typical build path of Nidalee as Fiddles and Diana will typically get by without RoA or another mana item, but they have to either keep taking Blues, or buy extra potions if they want to clear quickly and be relevant in lanes).
I don't like Magus. It needs a unique passive or different stats before I would consider it a good item.