How "intense" is this interview going to be? What, aside from my research interests, will I be asked? What are my chances of being admitted? I also am a little miffed at why someone would admit some students and then spend money to bring other students to interview.
Like everything else, it will vary by program, although I think the general rule is similar to what has been offered already.
In my experience, one program paid to fly me and other candidates in for a couple of days. We had a series of group and individual meetings, and I felt that once we passed the "crazy test" that the focus then became recruiting us into the program.
I am aware of another program to which a friend was "admitted." I use the quotes because in this case the school paid for all admits to come for a round of meetings and interviews. At the end of the weekend, only certain admits were offered funding. For this program, those not offered funding were essentially told, "Thanks, but you'll probably do better elsewhere."
If you really want to go to this school, contact the DGS for a name or names of current students. Then call one or more, as they will be able to give you a better idea of what to expect.