Soul coins are uncommon magic items, and are definitely more of a barter thing.
The value can fluctuate based on the soul they contain as well, as all souls aren't necessarily created equal. They'll also be much more sought after by individuals who need to fuel war machines.
— Dan Dillon 👥 (@Dan_Dillon_1) September 3, 2019
There'll be some other guidelines in Descent as to how you might use soul coins, how many various devils might accept in lieu of other services or payments.
— Dan Dillon 👥 (@Dan_Dillon_1) September 3, 2019
What @Dan_Dillon_1 said, but the rarity of uncommon is for Avernus. They are far more rare on the Material Plane which makes them highly sought after by diabolists, scholars, and evil collectors alike. Mmm, that's a good point. Much harder to get when you're not next door to the mint.
— Dan Dillon 👥 (@Dan_Dillon_1) September 3, 2019