A digital copy of ARR costs like 9 €. And they all come with a 30 day free trial. Even if you have to add another 1 month sub to it (which I don't think you have to), it would be like under 60 € for the mount.

And according to this picture, inviting a friend is already enough to be eligible for the first 5 feathers, adding another month of sub would only result in a few uninteresting items.