According to my testing with RatingBuster, I derived that one spirit equals to 0,564375 mp5.
According to other comments, this trinket gives 29805 mana on use, or 828mp5. Text below is updated to compare with this new number over the Wowhead's reported 17201 (or 477,8mp5)
Thus, we can compare Spirits of the Sun and Relic of Chi-Ji, previous top choices for regen. As the moment of writing this, 5.2 raid trinkets remain a mystery.
Spirits of the Sun provides an average of 967 spirit, or 545,8 mp5. Over the course of 3 minutes, it averages into 19647 mana. Less due to it not proccing instantly (altough, on-use trinkets may be left unnoticed, and, at the very least, unable to be procced midcast). Fully upgraded heroic Spirits gives an average of 1599,8 spirit, or 902,9mp5. Fully upgraded heroic spirits is better, which is no surprise since the ilvl is mere 5 off, and the other is heroic raid drop.
Relic of Chi-Ji provides average of 1210,8 spirit, or 683,3 mp5. Again, if we run it over the course of 3 minutes, it averages into 24600 mana, thus this new trinket is better than our Darkmoon card, unless you benefit more from short-term spirit procs (like pre-5.2 disc priest raptures, old mana tide..) over raw mana. However, if you have upgraded your Relic fully you can expect it to provide up to 1449,3 average spirit, or 818mp5. This Soothing Talisman will also hold 221 intellect more.
And since some raiders may prefer Qin-xi's Polarizing Seal over abovementioned, it gives flat out 1079 static spirit, or 609 mp5, going for 21922,6 mana over the course of 3 minutes. Qin-xi, fully upgraded provides 1312 spirit, or 740,5mp5.
(Added 9th of February
Now that we have some knowledge of 5.2 raid trinkets, we can now see that Blizzard is again deducting our possibilities to go for manaprocs - three trinkets, one of them proccing mana, one of them having static spirit and one pure throughput;
Horridon's Last Gasp will provide us 4780 mana every time it procs. However, ICD is unknown. Hopefully I will come and update this once we know the ICD. Throwing arbiturary number of 30 sec ICD (yes, it's odd number, I do not recall other trinkets having that number), it would provide 796,7 mp5. This is for the normal version.
Inscribed Bag of Hydra-Spawn provides static 1467 spirit, or 827,9mp5, with an intersting absorb proccing mechanic as throughput proc.
Finally, we have Lightning-Imbued Chalice as a full throughput option for those who deem themselves having enough spirit, and/or fight rendering spirit useless for reason or another.
TL;DR: Better than anything pre-5.2, with exception of fully upgraded Spirits of the Sun Heroic.