So here are the current factors considered for overall weapon balance.
These are the details of the current default settings:

Weapon range: Medium Close
Weapon can be fired up to range 14 with no aim penalty, highest bonus to hit is at a 4 tile distance.

Weapon Cost: Same as Rifle
restricted upgrades: Crossbows cannot use magazine size attachments or muzzle attachments. Optics, stocks, triggers, and custom attachments are permitted.

Weapon damage
Conventional: 4 damage, 1 Pierce, 1 upgrade slot
Laser: 5-6 Damage, 1 Pierce, +5 aim, 1 upgrade slot
Magnetic: 7 Damage, 1 Pierce, 2 upgrade slots
Coil: 8-9 Damage, 2 Pierce, 2 upgrade slots
Beam: 10 Damage, 1 pierce, 3 upgrade slots

Weapon attachments: 10/20/30 Supplies for tier 1/2/3

Templar Assembly - Requires Contacting Templars, +2 Damage vs Psionic Enemies , 1/2/3 Ablative,

Skirmisher Assembly - Requires Contacting Skirmishers, +1/1/2 Damage on Flank shots, +10/20/30 Critical hit chance on flank shots, +1 Crit Damage,

Reaper Assembly - Requires Contacting Reapers, +1 damage from concealment, +10/15/20 Aim and crit chance while concealed. Reduced Detection Radius.

Marksman Assembly - Requires Hybrid Materials, +15/20/25 hit/crit if the soldier did not move on the previous turn, +1 damage from high ground.


All attachment bonus damage applies to the weapon, any abilities which use the weapon, and any custom bolts fired from the weapon.


Bolts, quivers, and plating items are locked behind Proving ground projects - for balance and convenience, if you don't want to use the optional added features, just ignore the proving ground projects.

Bolts:
- Saboteur Set - Requires Proving Ground Project

Shardburst bolt - Small AOE explosive damage = weapon damage, each target in aoe is attacked individually as if you fired at them, hits are not guaranteed - good for opening on a pod where all targets can be easily hit.

Detonation Bolt - Detonates an explosive carried by the target, slight bonus to hit, inflicts normal weapon damage + damage from carried explosive, cannot target enemies not carrying explosives. Does not break concealment.

Breach Bolt - Significant bonus to hit, destroys cover on hit or miss, cannot crit.


- Assassin Set - Requires Proving Ground Project

Seeker Bolt - Significant bonus to hit, Debuffs target's mobility on a hit, does not break concealment.

Stunning Bolt - Applies 3ap stun to target, does not break concealment.

Overload Bolt - Deals significant damage + stun to robotic enemies


Disruptor set - Requires Proving Ground Project

Neurotoxin Bolt - If primary target is hit, creates a cloud of poison, has a small chance to stun and a good chance to disorient anything in the aoe, deals weapon damage to primary target, chance to deal poison damage to targets in the aoe.

Caustic Bolt - As neurotoxin but Acid instead of poison, deals significant shred to primary target.

Immolation Bolt - Line attack that has a chance to set fire to anything in its path, deals weapon damage to any targets struck, hits are not guaranteed.


Bombardier Set - Requires Proving Ground Project

Demolition Bolt - Deals significant AOE environmental damage, and a very small amount of damage to any targets in the area. Guaranteed to hit.

Concussion Bolt - Deals a small amount of AOE environmental damage, a very small amount of damage and has a chance to stun or disorient any targets in the area. Guaranteed to hit.



Quivers:
Quivers require a proving ground project and are unlocked with Plated Armor
Only one Quiver can be carried by a soldier
Each quiver provides one charge of the bolts associated with it, except the bombardier quiver, which provides 2 of each.

There is a second tier of quivers unlocked with powered armor:
The War Quiver and Warden Quiver, which provide 5 bolts instead of 3.

Plating:
Requires proving ground project.
Each quiver, except the bombardier quiver has a 'matching' plating.
If a soldier is wearing the Matching Plating and Quiver, the quiver provides an additional charge of each associated bolt.


Soldier perks:

If you're interested in using the crossbow with your custom class, you'll find a list of Passives in the X2Ability_SoldierPerks.uc that are associated with the XComXbowBolts.ini - feel free to add these perks to your classes xcom row or ability tree to enable additional crossbow abilities.