In any case, if sufficient units have sold to warrant demand, they can literally offer a monopoly on the market to anyone interested in firing up a plant. Heck, they could do it themselves and keep it all in house.
They need to sell enough to bring the demand up though, otherwise redesigning the software and hardware to work with the nMP wont be worth it.
How much would a redesign of a GTX780 or Quadro 6000 cost? $1,000,000 (* insert Dr Evil pose here *) ? Now you need to put aside resources, labor, and space at a factory. Then comes rebranding, marketing, packaging, negotiating, distribution, etc.
Plus there's opportunity cost: All these people doing these things have to be taken off other projects that may be more lucrative, by the way.
Then you have to convince a bunch of Professionals to give up their machines for a few days (and pay a fee) so the "Geniuses" at the Apple Store will install them, because only Apple, super-nerds, and serial killers have torx screwdrivers.
Sure, they could mark it up 100%+... but the volume is going to be very anemic.
Apple could fudge their software to cut out D300 users, but they can only push users so far before they just switch platforms. The nMP is already going to squeeze a lot of people out of Mac OS, how much more are they willing to lose just to make a few bucks off the people they have left.
Eventually the market will intervene--Microsoft will make a decent OS (Win 7 isn't the worst thing in the world) or some enterprising company will make decent GPU drivers and pro software for Linux. It will happen, eventually.
I think it's highly unlikely 3rd party cards will enter the market, given that I doubt the sales of the MP will pan out (3 grand for a Quad with 256GB? Please.). I think Apple will sell a bunch, but not nearly enough to attract enough 3rd parties to the GPU and professional thunderbolt market.