The Kashira is the pommel of the Katana which is placed under the handle.
What is the purpose of this?
This part of the nomenclature is used to attach the Ito to the handle and hold it in place. But beware, historically this small part of the Katana was not only used to hold or to make it look good, it happened that in close combat fighters used the stock to hit their opponent in the face.
The Kashira is strangely related to the Fuchi of the Katana. Traditionally they are made with the same materials and often represent the same work through the carved pieces. For example, you can find the folklore of the same Japanese demon on both pieces.
Generally the Kashira will be made of the same material as the other fittings like the Tsuba or the Fuchi. So you can find a Kashira made of brass, copper, iron or even silver! Some are very detailed with fine carvings, gilding and silvering.
You can expect to pay more than $100 to get the Fuchi, Kashira, Tsuba and even Menuki. Obviously, if you want antique or very high end, it will be much more expensive than that.
It can happen that with practice and cutting, the Kashira can become detached from the handle. So you have to be careful with your practice, but also with the environment in which the Katana is, because underneath the Kashira there is wood that can expand.
This is not a major problem, you just need to buy some glue and put a bit on the Kashira so that it doesn't move anymore.