Depending on the quality of the stamp it could be worth quite a bit of money. Also, these stamps have issuing markings upon them which are indicative of the rarity of the issuing as well as other important attributes. The value of this stamp is best determined in person or by a dealer.
This particular stamp,from the early twentieth century,was a common standard-issue stamp,and there are probably many still around.If in good condition,it could be worth $20.00 to $25.00.
You could be looking at a twenty dollar valued stamp there. It all depends on the condition of the stamp and if it's been used. If it has been used then the value would drop substantially. Persevering these things is difficult as they require dry places to be stored to keep the adhesive from activating and drying up.
If the stamp has been kept in mint condition, its value would be around ten dollars and seventy-five cents. If it has been used, however, the value would drop to around forty cents.
There are different types of Franklin Green one cent stamps actually, so it will be hard to get an answer without having any other info about the stamp. It could be worth pennies or much more. If it's in unused condition it increases the value.