Ammo: twenty arrows weigh 3 lbs and cost 1 g for 20.
An arrow looks like it weighs 328-410 grains.
The grains to pounds calculator, notes that this is roughly 0.0468571 to 0.0585714 lbs.
Multiplying that by twenty gets us .937142 to 1.171428 lbs... so, that's pretty close to a third of the weight of pathfinder arrows.
For the arrow-heads, they look like they weigh 5-17 grams.
The gram to pound transformer indicates that this is roughly 0.0110231 to 0.0374786 lbs.
That... really doesn't help us for in-game stuff, as that won't even come close to PF-arrows. We might presume that the quiver is about two pounds or something, and it's a fair guess, considering the weight of the only known quiver in-game, but the rules don't really bear that out, and who knows how a GM will rule it.
So instead, let's look at special materials.
Under mithril, it has a FAQ that notes,
For example, a steel dagger for a Medium creature weighs 1 pound. For its mithral equivalent, multiply that weight (1 lb.) times 500 gp/pound, or +500 gp. When added to the original item's cost (2 gp), the mithral dagger's total price is 502 gp.
Masterwork information, notes,
Masterwork Weapons
A masterwork weapon is a finely crafted version of a normal weapon. Wielding it provides a +1 enhancement bonus on attack rolls. You can't add the masterwork quality to a weapon after it is created; it must be crafted as a masterwork weapon (see the Craft skill). The masterwork quality adds 300 gp to the cost of a normal weapon (or 6 gp to the cost of a single unit of ammunition). Adding the masterwork quality to a double weapon costs twice the normal increase (+600 gp).
... which is basically 2/100ths of the masterwork price.
Similarly, alchemical silver notes,
Ammunition: +2 gp (each); Light weapon: +20 gp (each)
That's about 1/10th the price.
Mithril is 1k/light weapon or 500 g/lb.
So either we go with something like 20g/ammo piece (like masterwork pricing), 100 g/ammo piece (like silver pricing), or the poundage, which is hard to determine... but is roughly a tenth of the weight of the arrow on the "high" end. Since an arrow weighs 3/20ths of a lb (in Pathfinder instead of real life), we can multiply 500 times that to get 1500/20 or 150/2 or 75 gold for each mithril arrow. If you're using closer to "real life" weights, but otherwise following PF rules, it'd be about 1/3 of that, or about 25g/arrow.
Hope that helps!
