Basic systems such as small rails (20mm is standard) with holes machined in them to be screwed onto the existing hand-guard of a rifle can cost as little as US $25 to US $40.
Rails, typically measuring 20 millimeters in diameter, are commonly found on rifles and can be purchased for as low as US$25 to US$40. These rails have pre-drilled holes that allow them to be attached to the existing handguard of a rifle.