The service will be free for a short little more than a month (December 27 - January 31)then you will have to pay a yearly fee of $5.
http://venturebeat.com/2013/12/13/pokemon-bank-will-hit-3ds-eshop-in-two-weeks/
Update: Pokemon Bank has been delayed
http://venturebeat.com/2013/12/13/pokemon-bank-will-hit-3ds-eshop-in-two-weeks/
Update: Pokemon Bank has been delayed