Yep, great mechanic. I'd recommend capping a quiver at a d10 ammunition die though. That's around 28 shots from a full quiver on average, pretty close to the 30 you get from a quiver of 20 arrows and recovering half of your used arrows after each combat, as per RAW. Each die size increase would cost 25 SP, so going from empty to full costs a total of 1gp, same as the cost of 20 arrows. Crossbow bolts work the same. Sling bullets cap at a d10 die and cost 1cp each.
Blowgun needles are a bit trickier, since the 50 needle bag doesn't translate as evenly. But if you let it go up to a d12 ammo die (about 40 uses on average) and charge 5 sp per die, two full bags ends up costing 5 gp. So carrying two bags gets you 5 more shots on average than RAW for the same price.
If you allow firearms, cap them at a d6 (10 shots on average) and charge 15 sp per die. Technically that cheats the player of 5 shots per pouchfull of bullets, since 2024 RAW fails to specify that firearm ammunition is destroyed after use, so by RAW it can be recovered same as any other ammunition. But that's obviously an oversight since they copied the firearm and ammunition pricing from the 2014 DMG, which does specify that it's destroyed. If you'd prefer to stick closer to 2024 RAW for some reason, give them a d8 ammo die and charge 1gp each. They get 3 bonus shots for the same price as RAW that way, but close enough for government work.