As you can tell from the photo, this is a worn coin. All denarii of Gaius (Caligula) are scarce, and some are harder to find than others. Denarii of Claudius are also scarce. The speculation is that after Nero debased the denarii, people hoarded all of the good silver coins, and this included denarii of Claudius and Gaius. According to Gresham's law bad money drives out good money. However, this does not explain why there appears to be plenty of earlier denarii available of figures such as Tiberius and Augustus but very few of Claudius and Gaius. We may never have a satisfactory answer.
Now why do I call him Gaius. Caligula (meaning little boots) was a nickname given to Gaius when he was young and travelling with his father's (Germanicus) army. According to contemporary or near contemporary accounts he detested the name. If you were emperor I am sure you would not want to be called "Bootykins".
The reverse of this coin has a portrait of Agrippina the Elder , Gaius' mother. She reportedly starved herself to death 4 years before Gaiusbecame emperor.
Caligula and Agrippina AR Denarius, aF, toned, bumps and marks,
(17.84mm, 2.680g) 180o
Lugdunum (Lyon, France) mint, end of 37 - early 38 A.D.;
Obv: C CAESAR AVG GERM P M TR POT (counterclockwise), laureate head of Gaius right;
Rev: AGRIPPINA MAT C CAES AVG GERM (counterclockwise), draped bust of Agrippina Senior (his mother), her hair in a queue behind, one curly lock falls loose on the side of her neck, RIC I 14 (R) (Rome), RSC II 2; BMCRE I 15 (Rome), BnF II 24, Hunter I 7 (Rome), SRCV I (2000) 1825 RSC 2Ex: the Jyrki Muona Collection, Ex: Forvm Ancient Coins.