Are the costs of each subscription per project or app?

In Apphive, the cost of a subscription is per project, it means, if you have one or ten apps in your project and you paid for a Premium or Unlimited plan, you can publish all your apps in this single project; and, if you have a Starter plan, all the apps within your project will have a private development. Just remember that all the apps from a single project share the same database.