How high is the maintenance for roads (outside of your territory)?
Based on experience, it appears that you only have to pay 1G for each hex of roads that are inside your cultural borders. That means that you don't have to pay to maintain a long road to a city-state if it goes through neutral land
Is this correct? Do I only pay for roads that are inside my borders?
(I think this factor is pretty important when settling a new city further off to get a luxury resource, isn't it?)
You don't have to connect your city via a road in order to get the benefit of a luxury resource. If the resource is in your city and the tile is worked you will get the resource. Reagrdless of having a road. Connecting the city via a road helps with a trade route and troop movement however.
As far as I can tell having a road outside your territory does not cost any gold to maintain.