From what I've researched, transmute specialization is 15% extra gems in the long term.
20 percent would be nice, and that's pretty close to 15, so that could easily be the correct number.

And that is pure profit, as there is no extra material costs involved.
And you get anywhere from 1 to 4 extras when it procs.

I'm sure you'll remedy the lack of x-mute specialization shortly.
Extra gold for free, is nice.