Historical notes

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 rec.pyrotechnics and an interesting post that "cribbed" the New Scientist regarding something called the Barrel Boom:

Gunpowder barrels were made of oak by a tight cooper and were actually good enough to hold liquid. The hoops surrounding the staves were of copper and wood, which are non-sparking, never of iron or steel. Standard full barrels contained 100 pounds of gunpowder, half barrels 50 pounds, and so on down to 2-pound barrels for sportsmen. When filling the barrels, space would be left to allow the powder to move freely. Regularly rotating the barrels would then prevent the powder from aggregating during storage.

So, one might be able to conclude a two (2) pound barrel is a "small keg" for the "sportsman" (or typical WFRP gunpowder user).

WFRP notes

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 is as follows: Poor 45s (1s 120p x 30, 2gc 5s), Common 120s (3s x 40, 6gc), Good 450s (9s x 50, 22gc 10s), and Best 1800s (1gc 10s x 60, 90gc).

Zeiss Ikon notes below that 233 grains would "be excessive" and further notes a typical .69 caliber musket—let's say this is a Hochland hunting musket or typical Empire issued blackpowder weapon for argument—would have a load adjusted between 80-120 grains. The gun owner, factoring any number of inputs including determined quality of the powder, experience with the weapon, distance to target, wind, even desired impact to measure an individual load. This research changes the number of shots per two (2) pound barrel (see Zeiss Ikon's calculations below).

Using Zeiss Ikon's calculations the costs are adjusted as follows, where the median of 120-180 grains is used, or 150, then 90 shots per two pound keg, at 3s per shot: Poor 90s (1s 120p x 60, 4gc 10s), Common 210s (3s x 70, 10gc 10s), Good 720s (9s x 80, 36gc), and Best 2700s (1gc 10s x 90, 135gc).

D&D 5e has abstracted this away into Lifestyle Expenses:

Lifestyle expenses provide you with a simple way to account for the cost of living in a fantasy world. They cover your accommodations, food and drink, and all your other necessities. Furthermore, expenses cover the cost of maintaining your equipment so you can be ready when adventure next calls.

(from Player's Basic Rules, p52)

You can describe this as including the payments on a house loan, if they want to buy rather than rent. And if they decide to sell up and move, tell them the money they made on selling the old place goes towards paying for the new place.