Another option that no one has mentioned is jobsite tool storage boxes.
Depending on your application, this may be a less obvious way to store a firearm than a gun safe that screams, "Guns inside!"
Tool storage boxes are somewhat secure if locked and chained in place, but are slow to access for primary weapons.
They are relatively inexpensive compared to a real gun safe...although, clearly, they don't offer the same protection.
However, one caveat though, storing firearms in jobsite tool storage boxes may be specifically forbidden by the manufacturer and will void the warranty...but do you really need a warranty on a steel box? Use at your own risk.
They are sold at Home Depot, Lowes, Grainger's, and other construction supply houses.