Yeah, I'd say $19 is really pushing it for this sort of app, which is something that is zero-maintenance, zero-interaction, install and forget kind of thing. The app needs to have some sort of presence (perhaps it does, I would not know, I am on Windows) that would prove that it is alive and doing its job. At the very least I would expect some sort of subtle visual feedback in the event when the blocked app is actually prevented from getting out. Showcase it in action on the landing page too.
The way it stands, it looks like a pretty UI for configuring built-in firewall. And $19 for that is an overkill. Also, keep in mind who your audience is - (paranoid) geeks, so it is important to show that the app has some non-trivial content to it and it's not just an empty shell on top of what's available for free already.
In other words, if you manage to convince me that the app actually has some brains and smarts, that while it is simple, it is not trivial, then $19 is a fair price. If not, then even $10 is an overkill.