I don't think current gunpowder prices are anything out of the ordinary. If you disagree you can always make your own through the alchemy crafting system too.
I can't get IG right now to check figures, but really it's the number of charges per bag and also the bag's weight. Rather than lowering the price (necessarily), I'd advocate for having more uses of powder per bag.
From the previous thread's discussion this is supposed to be black powder. Consider that armies would use this powder, recruits would have to drill (practice) with it, and it's also used in cannons. It doesn't seem realistic that powder would be so expensive and a military able to use it rather than arrows or bolts, given the relative handiness of each in game.
In the past couple of weeks I've come across strong alchemical bolts/arrows for between 300 gp and 450 gp. These items are crafted from rare essences, rather than ordinarily occurring minerals, yet when you divide them per shot the cost is much lower than for a manufactured bullet and powder. In game, they weigh nothing.
Looking online for RL numbers, I see that the charge of black powder required to launch a 45 caliber lead ball is 70 grains. Converting to metric, that's 70 x roughly 65 milligrams or less than five grams. A pound of powder, therefore, should be enough to fire more than 90 bullets.
At the current weight and prices, I don't think it would be practical for a government to equip its infantry with black powder muskets or use cannons.
Be that as it may, I would like to talk generally about guns here and whether there's any desire to improve them or they're just going to remain sort of a novelty. In terms of game balance, for example, if they don't have any feats or other ways to get better at them, then that severely limits how useful/balanced they'll ever be: they can't be very powerful or they'll be too strong for low levels to use, but if they're balanced for low levels, then high levels will have no use for them.
It would be preferable if there were some way (as with most other game features) that a character could through time and focus specialize as a firearm wielder.
The firing process is painfully slow and awkward even with two guns that on a very high appraise character cost more than 20,000 gold (which is pretty high for NPC merchants). Then there's the aforementioned bullets and powder. Since I still know of only one area that sells those supplies, you must also stock up on them, despite the weight of the powder.
These are a lot of constraints. Yes, it's pretty satisfying when it works and you blow someone's head off, but it doesn't happen very often. I don't think it's practical to solo as a gun specialist, and in a group the rest of the group are going to have killed everything before you get to contribute very much.
None of this means I think guns have or ought to be improved. Perhaps they're not even appropriate for the setting. But in reading the old thread it struck me that they once generated a lot of excitement and now they're kind of a very obscure novelty.