B&H Photo has the IRIX 15mm f/2.4 Blackstone lens for Pentax K-mount for $419 ($180 off) as a part of their daily deals:
This offer is valid for today only (December 5th).
Additional information on the IRIX 15mm f/2.4 can be found here:
New Irix 15mm f/2.4 full frame lens for Pentax K mount announced