From someone who has 34 alchemists..

Sure I'm close to 800k now, and am +400k since the launch of Cataclysm.
The alchemy definitely makes a lot of gold, over time.

But there's no way I log into the game every day and consistently do all the transmutes.
There was a period where I wasn't excited about a new team yet, and they had announced nerfs that would kill my ferals as a competitive team.... so I did my Alchemy consistently for 45 mins a day, as my subscription was still going for 3.5 additional months at that point.
That was probably the longest period of consistently doing the transmutes on all toons, every day.

The other thing...
My auction toon averages 8g per Volatile Life purchased.
And averages 18g70s per Volatile Air sold in 1's, 5's or 10's, and 22g per Volatile Air sold in 20's, 50's or 100's.
But when your transmutes supply you with more Air than people are buying, the stockpile doesn't earn you anything.
I've had periods where I have 4000 Volatile Air in "stock", either because the market was flooded with 9g Volatile Air for a week by some idiot who listed in a larger volume than I wanted to buy out, or whatever...
On my server, I average 10-12 stacks sold per day listed as 5's and 10's, and 4-5 stacks (of each size) per day listed as 20's, 50's or 100's.

I'd look at Alchemy as a means to make consistent levels of gold.
Which over time, form a very respectable passive income.
But not something that can and will always make... a x b x c = d.... gold per month.