Own all Weapons Attachments
You unlock weapon attachments by completing the main story, finishing sidequests and finding collectables. To buy an unlocked attachment, simply go to any vendor, choose "Weapons" and then select each individual weapon to view its purchasable attachments. There are 25 weapon attachments in the game, and they are quite pricey. Therefore, it is highly recommend to loot every enemy and container you can and to refrain from purchasing anything from the vending machines except for attachments and collectible maps.

The following is a list of all 25 weapon attachments and their respective prices:

Fazertron

Light Barrel - $750
Grip - $1000
Extended Mag - $1100
Full Auto - $2000
Even Larger Mag - $1500
Mag Pouch - $2000
Laser Rounds - $3500
A.J.M. 9

Suppressor - $400
Laser Sight - $1200
Larger Mag - $1500
Max Mags - $2000
Galleria 1991

Extended Mag - $750
Double Barrel - $1000
Semi-Auto Upgrade - $1500
Really Huge Mag - $1750
Bandolier - $2000
Quadruple Barrel - $2500
Fire Rounds - $3750
Kobracon

Extended Mag - $1000
High Powered Scope - $1000
Fog Scope - $1250
Aim Stabilizer - $1500
Big Ol' Box of Ammo - $2200
Semi-Auto Upgrade - $2750
Explosive Rounds - $3250
If you have done everything in the game but still need money to purchase attachments, you can use this method to farm money post-game: