I think it's $100 mil because she was Jane Birkin and that's why it's valuable. It has nothing to do with the bag at all. It's like would you pay $100 million for a Van Gogh painting even if you hate Van Gogh? Of course you would because it would be a bargain on a valuable investment.
I think of my ex when it comes to purses. She spent most of her life searching for the perfect purse. Seriously, I must have waited in hundreds of purse stores over ten years. Mankind built the Pyramids and walked on the moon, but according to her, we never made the "perfect" purse.