From what I've gleaned from people "in the know", this is EXACTLY why we haven't gotten sequels to the previous TCM duology, the Friday & ANOES remakes, and Zombie's Halloween 2. They cost too much to justify spending the same on a sequel that will likely have diminishing returns. That's why the classic franchises remain dormant for now and shit like Saw & PA thrive. They are cheap and even if they disappoint, they still bring in the $$$ like crazy.
The difference is this case is that Lionsgate/Twisted Pictures have apparently only spent $20 million on Texas Chainsaw 3D. Now I realize that isn't too far from the budgets on the above, but they don't seem to have spent nearly as much on marketing as that set of films. As long as it manages to pull in the $50-70 million that recent similar "iconic" fare has, it will at least garner one follow-up.
EDIT - For anyone wondering, the ONLY reason we didn't get a sequel to My Bloody Valentine (2009) is because the producers didn't bother to purchase sequel rights along with the remake rights. Of course, once it became a moneymaker and they realized their mistake, the rights holders were demanding more for them than they were willing to pay.
For what it's worth, Todd Farmer's a pretty good friend of mine, and that's not what I heard about either Halloween 3D or My Bloody Valentine 2.
The Weinstein Company is broke. They had the money to make Halloween 3D or they had the money to make Scream 4; not both. And Lionsgate thought MBV's gross was a fluke and they didn't want to see a massive drop-off with a sequel.