I'm currently a concept artist, but would like to know if the troubles and stress of being a matte painter is worth the effort involved.
Why would you think that a matte painter would earn more money than a concept artist? If anything, it would be the other way round.
And why would you think it's more trouble and stress?
There is a huge amount of crossover between concept art, matte painting, 3D, texture work and compositing. And because there is so much crossover, I've never really noticed any massive difference is rates between the different disciplines.
But yes, as a really rough guide, (A) is probably a good number.
If you're going to choose this career for the money, you're on to a loser from the word go...