Well, Robin seem to sell bout 50k, which is more or less Nightwing numbers right now, and significantly sold more issues than Jon (34k) in September (based on comichorn)
Maybe we almost never see Robin on the ICV2 list as the list is released only a few days after the release date of the 4th week of the month. By this time people have almost 3 weeks to buy Jace's book, but leas than a week to buy Robin.
Anyway, right now Jon sales are very good. But would this trend remain in 3-4 months from now?