The Old World Armoury for Warhammer Fantasy Roleplay (second edition) says that gunpowder is sold in "small kegs" and costs 3s per shot, but how many shots are in a keg? I can't find the answer anywhere in the book.
After some light reading in several top ranked Google results, a phrase buried in the history of 18th and 19th century British artillery pages stood out: "dry and tight cooperage."
This in turn led me to the usenet group
A two (2) pound barrel is a "small keg" for the "sportsman" (or typical WFRP gunpowder user).
The Old World Armoury, as noted, states gunpowder is 3s per shot. The challenge then can be mathematically abstracted for WFRP: How many shots in a 2 pound small keg? Further research about blackpowder development and weapons suggests that the quality of the powder factors heavily and modern gunpowder suggests "about 30 rounds of .45 caliber per pound" (one pound = 7000 grains and a typical .45 load ≅ 233 grains.)
Most 16th–18th century long rifles and pistols had a median caliber of .50, making the .45 is a good measure for abstraction. If modern powder translates to Best Quality then 60 rounds per "small keg." Reduce by 10 for Good, 20 for Common, and 30 for Poor qualities, or 50, 40, and 30 rounds, respectively.
The cost then is a based on the quality scale with 3s per shot with Common quality gunpowder, or 3x40, 120s (6gc), 450s (20gc) for Good Quality, and 1800s (90gc) for Best Quality.