In answer to "why it's like this" in terms of vendor/crafter disparity it basically amounts to a lack of planning and cohesion in design. The disparity exists because vendor item values are determined by a set of 2das that multiply a bunch of numbers together based on the item type and what properties it contains. We didn't make it this way intentionally, and we didn't adjust the values in these 2da files at all. When area builders first put those Heal potions on vendors they should have added a fixed cost increase to them to make them equal in price to a Heal scroll, but the fact of the matter is that the designer didn't sit down and compare the cost of a Heal potion to a crafted scroll--neither Obsidian nor our own area builders.
The honest truth is when you're building a module things slip through the cracks and not everything can be adequately planned for. The cost should be the same (or the potion should cost a bit more), that much is certain, but it's the price of the potion that should go up, not the other way around. Being able to carry an infinite supply for 800 gold each is pretty ridiculous both mechanically and in terms of lore, they should probably cost four or five times that.
That being said, it is what it is and it's been like that for years. If I recall correctly we already lowered the cost of scribing scrolls from 25 to 15 to account for this "vendor disparity" and we're really going in the wrong direction at this point trying to decrease it further and further--we simply need to go through the vendors and raise the prices on things so that crafters can be competitive. On the other hand it's a tedious job, to be quite frank, as there are a
lot of vendors in our module, so it may take some time, but I feel like it would be better than lowering the cost of crafting consumables further.
I have to say that I find the notion that gold is difficult to come by is unfounded, I spent 30 or 45 minutes on the Boareskyr Bridge killing the level 1 bandits yesterday and picking up their lootbags and sold the stuff to a vendor for 5000 gold and this is far from the most optimal scenario for earning money. At the end of the day BGTSCC is a game, and if a player doesn't really have the time/inclination to play the game and go kill monsters for an hour or two to fund his or her endeavors then that's not really something we can (or even should) solve through module changes.
So as far as I'm concerned, the cost of vendor supplies need to go up--not the other way around.