its another original sin inclusion tbh.
Arrows should be tracked in theory, although I dont necessarily mind them doing away with that for infinite basic arrows, its a CRPG afterall.
Elemental arrows as items isn't a 5e thing at all, there are spells that let you temporarily enchant existing arrows to be fire arrows and the like though.
That being said, a custom magic item that was a single fire arrow would probably be a common magic item, and its a consumable.so would be (1d6+1 * 10)/2gp, but should probably only add +1 fire damage to regular attack. a +1d4 fire arrow might be uncommon and would cost (1d6*100)/2 gp (so 150, not 300) based on D&D rules.
Nope. Effect arrows not only predate Original Sin, they predate Larian. In fact, they existed in the original BG.
The short answer to the OP, however, is that you don't? The only effect arrows I have were looted from enemies that had them, but didn't get a chance to use them on me. Even with that, I have yet to use any of them. There's a certain cave where you can get 4 Acid Arrows, and my ranger managed to pull 3 of them. I don't need them, but when I get to a tougher fight, I'll be sure to use them since I have them.