You should at least have some idea and no it can’t be from the peanuts you’re paid to run ads. Think bigger as people will open their wallets when they see value in your offerings. You can have a freemium app, but you’ll need to offer some feature or service that users can obtain through either in-app purchases (IAP) or in-app subscriptions (IAS) to generate revenue.
For apps that are designed to augment a business, like a healthcare booking app, you can measure value in terms of metrics like retention rates. Just remember, an app isn’t going to bring about a miracle if it’s not adopted and used accordingly, nor will it solve underlying issues with your organization. If people are leaving your practice because they’re still spending too much time at your office because you try to squeeze in too many visits or accept walk-ins to the point of significant disruption to your scheduled patients then don’t blame the app.