As with anything collectible, the value of an old magazine is determined by the demand for it. The best way to determine the value of a collectible is to consult with a reputable dealer.
The condition of a magazine is also important in determining its value. An excellent copy of a particular title will be worth more than another copy in poor condition. The size of the print-run is also a factor in value. Even magazines with controversial pictures or issues may not have any monetary value if there are many copies available. Issues of National Geographic tend to have a low value for this reason.