Let's do a comparison.
Morellonomicon
2100 g
80 AP
100% base mana regen
20 CDR
Grievous wounds passive (currently unique to this item)
Athene's Unholy Grail
2700 g
60 AP
25 MR
50% base mana regen
20 CDR
15% mana on kill, 1.5% of missing mana per 5
So, for 600g less I get 20 more AP, 25 less MR, 50% more base mana regen, and the grievous wounds passive. Sounds good to me! Remember, a null-magic mantle is 25 MR for only 500g.
"But what about the athene's passive?" you might think. I'm going to forget about the 15% on kill or assist, since after a teamfight you're either dead or pushing objectives, and mana is probably not a huge issue. So, is 1.5% of missing mana per 5 better than an extra 50% mana regen? Let's look at around the earliest one could acquire these items, let's say, level 7. In patch 4.20, almost all mages were normalized to start with 6 mp5, adding (growthstat)0.8 mp5 per level, so at level 7 they would have approximately 10.4 mp5. As for starting mana pools, not every mage is equal, but Ziggs, Syndra, Orianna, Ahri, Lux, Leblanc, Annie, and Zyra all start with 334 mana and a growth stat of 50. At level 7, that's a base mana pool of 586.75. 1.5% of that is just a smidge under 9 mp5, which is a grand total of 4 mp5 better than Morellonomicon's 100% base mana regen. When you have no mana at all.
TL;DR: the wiki says Athene's is 103.6% gold efficient, and Morellonomicon is 147.2% gold efficient, without taking passives into account. If you're an AP mid who needs MR, you might as well spend an extra 300 gold on
and help your team out.