A lot of this weapon's effects are easily and inexpensively duplicated or bettered by spells, and, as careful spells selection is integral to the game, I am challenging the frame here a bit.
The weapon's overpriced... if the assassin's familiar with magic
Such a weapon would be a boon to a mundane assassin with no ranks in the skill Use Magic Device. KRyan's answer already assesses the weapon under such circumstances in meticulous detail. However, many assassins—especially high-level, successful ones—are going to be casters, invest lots of skill points into the skill Use Magic Device, or both. Among such assassins, most of this weapon is significantly overpriced.
For all the extending of sneak attack distance the weapon provides, a +1 distance (DMG 224) (+1 bonus; 0 lbs.) heavy crossbow (PH 115, 116) (8,350 gp; 6 lbs.) with a wand chamber (Dungeonscape 30, 34) (100 gp; 0 lbs.) and a wand of sniper's shot [div] (Spell Compendium 194) (1st-level spell at caster level 1) (15 gp/charge) is far more affordable at a total price of 9,200 gp.
The spell sniper's shot—which, in addition to being a 1st-level Sor/Wiz spell, is also a 1st-level assassin spell—takes a swift action to cast then for 1 round
Your ranged attacks made before the start of your next turn can be sneak attacks regardless of the distance between you and your target. You must still fulfill the other conditions for making a sneak attack against the target.
If that's enough, skip to the Elephant in the Room, below.
If that's not enough—and were this my assassin's weapon of choice, it wouldn't be—also install a crossbow knife bayonet (Complete Scoundrel 109) (2 gp; 1 lb.) with a wand chamber and put within a wand of extended guided shot [div] (SpC 108) (1st-level spell modified to a 2nd-level spell at caster level 3) (90 gp/charge).
The spell guided shot—which isn't an assassin spell thus requiring a non-ranger, non-sorcerer/wizard assassin to make a Use Magic Device skill check (DC 20) to employ—takes a swift action to cast then for what is usually 1 round (but in the case of the wand, above, 2 rounds)
...your ranged attacks do not take a penalty due to distance. In addition, your ranged attacks ignore the AC bonus granted to targets by anything less than total cover, and also ignore the miss chance granted to targets by anything less than total concealment.
In sum, the weapon has its normal range increment of 120 ft. doubled to 240 ft. but can fire out to 2,400 ft. at no penalty via the effect of the spell guided shot, and the assassin can deal sneak attack damage at that distance via the effect of the spell sniper's shot. This is probably overkill.1
The price for the whole shebang is 15,302 gp.
For scale, the long edge of battle mat I use (which has 1½" squares to accommodate Lego minifigs) has but 31 squares, so this weapon's first increment is beyond the usual map range in my campaigns, and, I expect, beyond the map range of many others' engagements. Such a weapon is narrative device, suitable for explaining events that happen within the story, rather than an actual, practical hey-let's-go-explore-the-dungeon! kind of weapon.2
The Elephant in the Room: "That's a lot of darkvision"
Seemingly so that (mainly and, often, only) humans must bear torches, the game places an unreasonably high price on the extraordinary ability to see in the dark without visible light. Despite the majority of creatures in the Monster Manual possessing darkvision, we humans must either pay through the nose for such an ability or accept that we might be eaten by actual grues when the lights go out.3,4
Most creatures have darkvision 60 ft., and most items granting darkvision grant darkvision 60 ft. Getting darkvision 120 ft. or more using the tools the game provides is expensive... perhaps too expensive. (Seriously, by the game's standards and with no limits on its use, the original weapon's most expensive part is its magical telescopic sight.)
A wand of superior darkvision [trans] (Unapproachable East 53) (4th-level spell at caster level 7) is 21,000 gp, and for 1 hour/level 1 touched creature gains darkvision to the extent of his normal vision. It's what the super-sniper wants, and he pays excessively for it. (Although a Sor/Wiz spell, superior darkvision is also a 4th-level ranger spell, which might at least make using such a wand slightly easier.)
Alternatively, a liberal DM may permit a creature successfully making a series of Use Magic Device skill checks to benefit enough from Cannith goggles (Magic Item Compendium 84) (13,000 gp; 0.5 lbs.) to gain darkvision 120 ft. for 24 hours, but that DM's likely the exception not the rule. Also, the throat-slot item eye of the beholder (Dragon #340 69) (8,700 gp; 0 lbs.) for 1 min. 3/day on command grants, among other benefits, darkvision 120 ft. Otherwise, continuous darkvision 120 ft. appears to be the sole province of the robe of eyes (DMG 265) (120,000 gp; 1 lbs.), an absurd luxury none can afford.5
Luckily, we have an inexpensive savior in the extraordinary ability low-light vision. Low-light vision is deeply contradictory, but only in rare circumstances should an assassination need to take place in an area with absolutely no ambient light—not even the equivalent of moonlight or starlight—by which to view an unaware target.
So, while it will be useful on occasion for an assassin to be able to see in utter darkness, low-light vision should be enough for most murderers. If lacking natural low-light vision, there's the 1st-level Sor/Wiz spell low-light vision [trans] (SpC 134), which lasts 1 hour/level, and, conveniently, is also a 1st-level assassin spell. The 1st-level Sor/Wiz spell ebon eyes [trans] (SpC 77), although mired in controversy, might also work; it, too, is a 1st-level assassin spell.6
If the sniper is assassinating drow nobles in Erelhei-Cinlu or whatever, then it's time for that wand of superior darkvision. Until then, low-light vision (through whatever means) and a wand of darkvision [trans] (PH 216) (2nd-level spell at caster level 3) (90 gp/charge) should be sufficient.7
- The imprecisely duplicated version of the originally posted weapon costs 9,200 gp for the weapon and accessories plus 750 gp for a wand of low-light vision and 4,500 gp for a wand of darkvision for a total of 14,450 gp.
- The extreme-range version of the weapon—useful for murdering svirfneblin aristocrats in the Underdark—costs 15,302 gp for the weapon and accessories plus 21,000 gp for the wand of superior darkvision for a total of 36,302 gp.
In both cases, bolts and poison not included.
The assassin's extraordinary ability death attack can usually only be used with melee weapons. The 4th-level assassin spell sniper's eye [trans] (SpC 193-4) for 1 round/level grants the caster the ability to make sneak attacks at up to 60 ft. and specifically allows death attacks while using a ranged weapon... up to a maximum distance of a miserable 60 ft. Nothing to my knowledge extends this death attack distance even more, but a very generous DM may allow the combination of sniper's eye and sniper's shot to do so.8
An extensive assassin's handbook—including information about about death attack—can be found here.
1 The 2nd-level ranger and assassin spell near horizon [div] (Complete Mage 110) for 3 rounds only eliminates all range penalties, and you've ranks in the skill Use Magic Device anyway, right?
2 Technically, the wand chambers are unnecessary, but without them the assassin must drop or rapidly stow the wand of extended guided shot on one turn, drop or rapidly stow the wand of sniper's shot the next turn, then, on that same turn, perhaps also draw but absolutely make an attack with the +1 distance crossbow. The wand chambers, while a luxury, make this normally clumsy series of events far more elegant.
3 Complete Arcane details grues (153-5).
4 Not that I'm bitter or anything.
5 High-end intelligent magic items (DMG 269) can also have up to 120 ft. darkvision, although such an ability is of little use to the wielder in combat.
6 The Dragon #322 article "Patterns of Shadow and Light" presents in its original and slightly clearer form the spell ebon eyes (72).
7 Frustratingly, the only spell on the assassin's spell list that grants darkvision is, actually, sniper's eye.
8 To just go ahead and make death attacks using a longbow instead, take all the way to level 10 the prestige class justice of weald and woe (Champions of Ruin 48).