The Sigma 35 f/1.4 is a factor, but so is the 24-70 II. Any prime in that focal length range that does not beat the 24-70 II is DOA. Most of Canon's fast primes in that range are good enough wide open where corner sharpness is not as much of a concern for shallow DOF portraits/applications. Having primes that beat the 24-70II would give prime users a reason for using primes for all types of photography, not just for shallow DOF, because right now the 24-70II is comparable/better than the 24L II, 35L and 50L at the same apertures.
Primes typically cost less than the zoom that covers the same range (except the original 24-70L), so I'm guessing that the primes would be less than 2300 at launch and less than 2000 in about a year.