In Nakamise Street, the main street that starts from Kaminarimon to the temple Senso-ji, you will find a lot of small shops all along. These stores sell all kinds of things and among them pastries.
We bought cakes maybe made for student. It had a queue outside the store of students who were buying 1 or even 2 cake boxes. There was a reduced price on the boxes for students and schoolchildren. You can also buy two cakes in individual bag for 100 yens.
These are small cakes of about 6-7 cm height. This is actually sponge cake filled with Anko paste , the Japanese red bean paste. It's a little sweet but not too much. The sponge is very light so that the cake is eaten pretty easily.
It's very good and cheap, so go ahead without complex :p