Green gems vendor for 5g.
Cut green gems vendor for 9g. (I smell nerfbat already.)
I agree with your nerf prediction, but if it isn't...
With cut gems being so expensive, they'd have to sell in the AH for much more than that to avoid losing money in fees.
This might be an attempt to prop up the old WotLK rares, as Cata Uncommons would have to cost more or the people making them would vendor them until it did.
With gems being so expensive, the supply of Essences will be much smaller unless it rises to the disenchant-for-profit point.
Finally, and Blizz has shown repeatedly that they will adjust this, the Saronite shuffle always produced alot of dust for each essence, if we're not mass-shuffling, the amount of dust vs essences in circulation might be lower.
Personally I believe some shuffle will be necessary. It always has been needed to keep up with demand for enchanting mats, and the amount of enchanting mats needed is similar to the past. I'd argue that the price of Cosmics was largely determined by the vendor prices of jewelry and saronite bars, and the new essences will be similarly priced based on cut gem prices or bar vendoring.
Your numbers don't take into account wowhead's prediction of 0.28 rares per stack of ore, I'm pretty sure that they will severely change the numbers even though wowhead lists their vendor prices as 3g/3.75g which is insane compared to uncommon's 5g/9g. Another argument in favor of an imminent price-nerf, but I think the rares will go up in value instead, because blizz hates crafted weapons/armor vendoring for more than their mats, and the perforator (6 Rare gems) vendors for over 24g
P.S. May I suggest the Obsidian Obligato
(a flourish of different notes played together)