For me it would require that I got at least 4 Greater essences from each DE to be more profitable than transmuting those 3 Carnelians to an Inferno Ruby
In the beginning of the expansion it was quite profitable though when you could buy Carnelians for ~10g each and essences were 100g+
Generally, even with the guild perk youre getting an average of 2 greater celestials from each spike. So as long as 3 carnelians are cheaper than 2 greater celestials, fill your boots.
That's how I made most of my GCs before 4.3, but since then the math doesnt work out. Its cheaper to buy the other gems (alicite, I'm looking at you) and craft rings then DE them. Takes a little longer, but your margins are higher.