Iphone car remote start plan?

So I want to get my Girlfriend a remote start/keyless entry/ panic alarm system for her car, and she recently got a new Iphone. I searched that they can install everything but it costs extra with your cell plan to have the app to do all this from your Iphone. I'm just wondering roughly how much the plan costs monthly to maintain it?

Its not added to your cell plan
its a separate fee through the remote start company, since they have to add cellular connectivity to the remote start device in the car

looking online, it looks like they charge around $5 a month for this feature

It shouldn't cost you anything. It's just an app you install on the phone. It sends a signal to the car. Just like a call or text. The car receives it and starts.talk to whoever is doing the install.