KWS MK I SABERDART Please see page 41 of the Endless Vigil Sourcebook
for details.
Encumbrance: 1
Hard Points: 1
Rarity: 9
Price: 1,600
Type: Energy Weapon
Categories: Ranged
Damage: 1
Range:Long
Skill: Ranged - Light
Qualities: Limited Ammo 1
Features:
May be loaded with 1 dose of synthetic neuroparalytic or any other
poison. Whenever wielder successfully attacks a target, even if no
damage is dealt, the target suffers the effects of the poison

looking at the book the 1600 is for the launcher (and I think) 1 dart. After you fire your 1 dart, how much do reloads cost?

Asking here because bounty hunters are the typical users of said weapons (even according to the fluff text).

Enter the unknown has a model 77 air rifle, it runs out of ammo on a despair (as normal), and puts the cost of a *clip* of smarttranq darts (page 40) at 500 credits (they require a average resilience check, failure = 5 strain + 2 strain per threat, on despair repeat the check next round with +1 difficulty, rinse repeat with subsequent despairs)

I'm going with the saberdarts (only requires a successful attack not dealing damage) with Raquor Venom (because they're a lot more effective, requires a daunting resilience check or staggered for 2 rounds, 2 points of strain per threat, and immobilized on a despair) launched from a Security s-5 heavy blaster pistol page 43 of keeping the peace which has the text :
"...it's under barrel dart launcher can be configured to launch solid projectiles of various sorts; one particularly common choice is a grappling hook attached to a cable..." note that it has medium range (at least this part of the weapon has identical stats to the grapnel harpoon launcher in no disintegrations which has medium range)

would 350 per dart +150 Raquor Venom dose = 500 credits per shot compared to 500 credits per clip of (the less effective) smarttranq darts be reasonable?

In this case the 1100 for the model 77 + 500 per 1 clip = 1600 credits is the same cost as a saberdart launcher which comes with 1 saber dart (both are long range, one uses ranged light, one uses ranged heavy)

Limited Ammo 1 is generally a one-use weapon. In the Limited Ammo quality description it says "This also applies to grenades and other 'one use' weapons which have the Limited Ammo 1 quality." Right before that section it states that "Some weapons fire particularly large or complex projectiles that cost significant amounts of credits, or are themselves a complete weapon that once launched is expended."

Based on those, it seems that each shot from a Saberdart is 1,600 credits. There doesn't appear to be any reloads for this weapon.

I didn't think I'd have to explain this but...

The dueling pistol in suns of fortune also has limited ammo 1 and is not a one use item. There's a fiver slug thrower pistol with limited ammo 5, which is reloadable.

The exact quote you referred to is "The Saberdart, a name that applies to both the dart and the launcher, is a tiny roughly triangular durasteel alloy dart fired via compressed air." That sentence taken by itself (i.e. out of context) could be interpreted either way but then there's the following quote on page 42.

But at the top of the next page it says "Saberdart launchers have lightweight barrels that propel their darts via compressed gas. Dart launchers can be built into gloves, pieces of armor, other weapons, even innocuous pieces of everyday gear like data pads and glowrods."

You generally don't build one use items into armor. Also the combined package (the dart and launcher) has enc 3 and one use items like grenades generally have enc 1, and "a tiny roughly triangular durasteel alloy dart" certainly wouldn't have enc 3.

So on the whole when that one sentence you referred to isn't taken out of context, it sounds like the reusable saberdart launchers/weapon-systems are referred to in universe as "saberdarts". But your mileage may vary.

I have a player with a bow making custom arrows and another one using hand made darts (disguising them as hair pins).

I charge 50 credits per basic arrow/dart + modified depending on what crazy stuff my players want to rig those with. when I have time for this between sessions I just take basic price for appropriate poison dose (e.g. from LoNH or elsewhere) or explosive and muptiply by rarity depending also on location where you are trying to get them. normally this would be around 100-450 per arrow/dart if bought in a black market shop on any less regulated world. if I need to come up with a number on the go it would be something around this.

I would go with the cost of whatever poison is used and leave it at that.