Yes, there is a subscription cost for unlimited access to Halfbrick+ .
A Halfbrick+ subscription is offered when accessing any game in the service. After subscribing, you'll be prompted to create an account, you need to complete that step to begin playing.
If you've subscribed and can't access a game, make sure you've created an account on the original game and device you first subscribed.
Not ready to subscribe? Each game will have free guest access period, indicated by a countdown timer.