Where are you seeing them that is costs that much? The most expensive ticket for the show I went to was like $60 for the ochestra pit and the rest were in the $35-$45 range. The one I got was only $40, i've never paid more than around $50 to get good seats for Dream Theater.
You bring up a good point about them not switching the setlist up but I don't think there were that many people going to multiple shows because of that. At least not enough people to sell a couple thousand extra tickets to each show. I didn't know that about why live nation was booking them in bigger venues but it does make sense if they own those places. Still, you would think that the band playing to venues that are half full or less would still cause live nation to lose some money with the lack of ticket sales.