Brief Summary of the Rank Difficulty Multipliers

If Rank 1 is 1,
Rank 2 is 1.1,
Rank 3 is 1.25,
Rank 4 is 1.5,
Rank 5 is 2,
Rank 6 is 5,
Rank 7 is 10,
and Bastard! is 25.

-Credit to Szara Kryik (and all the other helpful Bastard Rank masters)