The player's guide for a game will sell quite a bit, as everyone needs the rules.
The "splatbooks" tend to sell well as both players who need Type X will buy it, those who are completeists will buy it, and often one or two more in the group will be their own, just in case they want to make a blank sometime.
The GM book, by definition, only sells to GMs. This means that about one fifth of your main book's sales, if that, will go that-a-way.
In general, this is a bad thing, but the books are normally important enough that publishers push through it for certain games (like D&D) but not for others.
Me?
I'd love to put together a PDF-release of a GM book. We have a published GM screen, true, but a book that could walk a GM through a bunch of situations, explain some options, lay out how to do stuff... I think that there's a market for it. (Mind you, I'd also expect a small tornado of complaints form people who play differently, but, that's to be expected.)
But I'm totally down for it.