The key to breeding good bettas is starting with good stock. If you breed pet store bettas then you will get bet store quality bettas when you breed. It will be nearly impossible to sell pet store bettas.
I would imagine it would cost at least $30 - $50 for a quality pair of bettas not counting shipping.
Let's just say I tried breeding bettas once when I was younger. This project ended abruptly when my mother discovered that all of her bowls and tupperware were missing.