I don't think it's as cost efficient as you make it out to be.
Sure, on paper, it seems very good, maybe it is, and it's only 2900 (?) gold.
But you need to see what other items a mage can build and why in some cases, Morello isn't as important and I will use Viktor as an example, since he is my main.
Before the mage update, I usually went Viktor item / morello / lichbane / deathcap / voidstaff / boots. After the update and the changes to his laser, ult and Q, I had to get Rylai's, because of the delayed 2nd proc on the laser and the delayed ult procs, meaning I had to slow people down to get my whole damage off. This means I had to drop one of my other items. I still consider 40% CDR Viktor better than any other Viktor build, but I had to change my build to get 30% CDR (10% scaling from runes), because I dropped Morello for Rylai's, and it seems to do the trick better. I consider deathcap,voidstaff and lichbane core on Viktor, so it had to be Morello.
My point is that Morello is a really good item (100AP, 20%CDR, GV, mana regen, flexible build path), but there are other items that offer things that can be more useful in specific situations or champion. For Viktor, Rylai's + 30% CDR seems more important than Morello + 40% CDR. Azir doesn't get Morello. Old Ryze didn't get it either. Cassio, Kassadin, Anivia and so many other champs don't need it, because they either have other core items (mainly tear / rod of ages), which means that after those items, they need to go into voidstaff / deathcap, or there are other items who work better on them.
Let's say you have a Lux and somehow you get fed 1 or 2 kills. It's better to go for Luden's Echo, because the damage you deal with it is so much, that the Morello trade-off wouldn't be worth it.
It all depends on the situation and the champion. Sure, it's good as an overall item, but adjust the build path to whatever suits best. Personally, I loved this item, but lately, I've grown out of it, since I saw how powerful Rylai's is. I still build it on a lot of champs though, and occasionally still go for Morello instead of lichbane or Rylais, depending on the situation.