I'm currently trying to figure that out also. my guess is either those had a lesser run time and/or something completely "bizarre" happened. I just flipped open vol 7. and they are in some wierd amusment park so I really want to know want happened in vol.6.
I had a similar problem when I started collecting the Series Black God in fact it was vol 8. that was $50+ while I was collecting them I downloaded a fan translation and that one seemed to be the book where the anime and manga completely went separate ways there were little changes here and there before but damn. luckly after the series completed I got it at a normal price.
Vols 6 and 8 have actually gone down a lot now just a couple of wweks ago I swear 6 was $50+ and 8 was well past $200 on Amazon
and they should release the other arcs already all we got from the looks of it are Stardust Crusaders and Steel Ball Run
