Simple answer - around £300, and yes, this Polaroid holder fits any 4x5" with spring back.
Make sure you really want a Mark VIII - these have an on-axis front lens tilt which is theoretically better but the movement mechanism is not as robust as the earlier Marks. I had a Mark VIII and soon traded it for a Mark VII as the Mark VIII had metallurgical problems (embrittlement of the alloy [I presume duralumin] leading to thread stripping). MPP's glory days were fast receding by the time they built the Mark VIII. For comparison, check out how much you would pay for a Linhof Technika III.