How much is your bed sheet?

Mummy bought a new bed sheet for Topaz few weeks ago after Topaz sleeps in Topaz's and Orion's room. The bed sheet was choosen by Topaz himself- Tweenty bird's bed sheet as he did not want any animals' bed sheet as "they will bite me!!" said Topaz.

This was what Mama told Mummy.

Mama: Topaz, you got a new bed sheet?
Topaz: Yes. Mummy bought me.
Mama: How much is the bed sheet?
Topaz: hmm.. I don't know. Mummy paid for it....but it is not expensive.
Topaz said again: Mama, do you know what is not expensive?
then he continued: It means Mummy can afford it!