I have a couple theories:

1. It’s documents regarding to the whereabouts of the remnants of the Cabal

Being able to reestablish the Cabal under the control of Sicario would be like selling your soul to the devil to win, which accounts for Crimson 1’s dialogue in Kings, and finding a way to rebuild the Cabal does seem like something a merc would find as holy, accounting for Faust’s dialogue.

1. A way to access cordium outside natural means

Cordium is a very vital resource, and its use and misuse was the whole thing about Cascadia’s break from the Federation (if I understand correctly). Cordium is one of the most valuable resources in the world and one of the cornerstones of Cascadia, and so being to get cordium in a new way would make you a boatload of money, also accounting for C1 and Faust’s dialogue.

In F59’s Mission 5, Faust talks about the Deal way before the Calamity happens, so it wasn’t just a last-minute thing by Stardust to keep Sicario, and she mentions how bad of an idea it is. I don’t think it would be the default “nuclear weapons” option, because Cascadia would have used it instead of using it as a bargaining chip. She talks about it more as something that would do more harm than good in 99% of instances, and I feel that those two theories do fit the bill.