do you have to pay a monthly payment or sumthing if you have apple tv

you'd have to pay for a Netflix subscription, for instance. obvioulsy if you want to use Netflix or rent movies from the iTunes store, you need an internet connection. monthly fees may apply 😉

otherwise, you pay once for the unit and that's it.

There is no monthly fee. The Apple TV includes a wireless card to connect to your wireless router and network for internet access - to access YouTube direct from the Apple TV and the same for Netflix if you have a Netflix subscription, access to your iTunes library - music, movies, tv shows, and podcasts, and access to photos on your computer streamed from your computer that is connected to the same wireless network as the Apple TV, and access to the iTunes store to rent movies and tv shows available via the iTunes store.