The price of the Sigma 24mm f/1.4 DG HSM Art lens is $849 and is currently available for pre-order at B&H and Adorama.
Shipping will start on March 20th, 2015.
Additional information on the lens, including MTF charts and detailed specifications, can be found on here.
Quick price check: the Nikkor 24mm f/1.4 costs $2,000, the Sigma 50mm f/1.4 Art is $950, the 35mm f/1.4 Art is $900.