I'd guess more, maybe $15000.
Just look at the price of some of the Canon and Nikon super-telephoto, wide aperture primes out there. This lens is faster than most of those at the same focal lengths. It's also a zoom lens rather than a prime, which offers far more flexibility. Then you need to subtract a bit of money from what the Nikon or Canon equivalent would cost because it's a Sigma and not a Canon or Nikon. However, an expensive Sigma is getting quite close to the prices of equivalent Canon or Nikon........so maybe this Sigma is around 90-95% as expensive as the equivalent lens that Canon or Nikon offers. Too bad there's no equivalent right now from either company.
The Nikkor 400/2.8 is $7800, the 600/4 runs about $2k more, and the 500/4 about $600 less. Canon's long guns are cheaper, so we'll stick with that- that should put a 500/2.8 somewhere near a 600/4, granted a zoom will add to the price. In the past, Sigma hasn't been anywhere near 90-95% at the long end in pro glass- the Nikkor 300/2.8 is about the closest comparison we can make, the Nikkor is $4499 for the USA version and the Sigma is $2599 (B&H USA only- Canon is $3750.) Furthermore, the Sigma 120-300/2.8 is $2700. Granted, adding the LCD and TC are going to drive up costs, but I doubt by more than $1000.
I'd say at $12,000 and under, Sigma have a clear winner, but over that it's going to be difficult unless performance wide open is stellar. However if they can come in at or under a Nikon 600/4, as long as it performs well, they probably won't be able to keep up with demand for the first year.