Groceries, anyone?

Went to the grocery store yesterday.  They have special buggies with baby seats on them.  I got one and was strapping Sybille in.  Tassia started laughing, and said (or at least, I thought I understood)   Tassia: (laughing) By Sybille. ME: OK, I can put you in the cart with Sybille. Tassia: (laughing again, and this time, I understood why) Buy une Sybille!   Yes, but for how much?