there’s an early skip to get to the forbidden forest before even reaching cathedral ward, from there you can defeat shadows of yarnham, get to burgenworth, and kill rom which progresses the night to the red moon. if at any time after you go to cathedral ward and kill amelia, and interact with laurences skull, you can rewind the night to after amelias death, making products cheaper. some challenge runners do this after killing murgo’s wet-nurse to just have cheaper wares for things like challenge runs or chalice dungeon delving. as for prices before amelia.... data mining? i dunno.

To add to what the first reply said, the prices before Amelia are known because there is actually another time change trigger prior to Amelia that most people don't notice on a normal playthrough. When entering Cathedral Ward for the first time after defeating Gascoigne, a time change occurs during the cutscene moving the time of day from dusk, to slightly later dusk. The change is hardly noticeable normally, but by using the Forbidden woods skip to skip this trigger and then come back to it later, you can turn the game state from either Night or Bloodmoon back to dusk.

If you don't like it, don't go through the effort. It's that simple. Complaining about an item being week isn't useful. Btw, the reason it does little damage is because you're supposed to hit strong opponents with at least 10-15 of them, causing poison to stack. This will kill, or severely damage, large opponents over a minute or two. In other words, it's a desperation item. If you want to poison opponents in other ways, you can get cursed gems with poison and attach that to your weapons. Like the poison knives, they need about 15 hits to stack high enough. That bar you have show up when getting poisoned? The enemies have that bar as well. The point is to fill their bar.

Kind the point in-lore. They take a while to build up poison and then the poison takes a while to kill them. Only really good against humans, and even then just hitting them with a saw or sword would be faster. The doctors use it because they spend more time studying and experimenting than fighting, and even they know that it's a defensive tool at best.