...but you still need Amiibos.

It appears that the only cost of entry to the Animal Crossing Amiibo Festival is itself the Amiibo.

According to Youtube personality TheBitBlock, the game will be a free download that just requires the Amiibo to start.

BTW I found out that Animal Crossing Amiibo Festival is a free download game. Amiibo are required to play though. #E3 — Josh Thomas (@TheBitBlock) June 17, 2015

Retail bundles for the game are listing on websites, but the bundle has already been confirmed to include two Amiibo and three Amiibo cards.