Mostly depends on what you are going to buy, but if its just hentai doujinshi, I'd say a good average is 500 yen per book.
At least in my comiket experience, (went to C81 in winter, and 75 a while back) I'd say 500 is about the average.
If you get 'packages' from the higher tier circles (i. e. urushihara), its usually around 2000-3000 yen, but you usually get cool poster packs and such with his stuff.
So in general, 500 yen per book. There are people who discount their books near the closing time, and with less popular circles.>tfw otherwise very decent artists put out good content but no one buys them because they aren't well known.