A lot of the pricings in the DMG are pretty bad, IMC I usually find that everyone prefers stat-boosters and cloaks of resistance over most other items due to their comparatively low price. I've doubled the stat-boost prices and tripled the prices of cloaks of resistance and am fairly confident that players will still buy them.
As for +6 to all ability scores, I'm not sure that's worth 396,000 even for a monk. For that price you could get Bracers of armor +8, An Amulet of mighty fists +4, An Amulet of Nat Armor +5, a Ring of Prot +5, A cloak of resist +5, Belt of strength +6, Gloves of Dex +6, and still have change left over. 300,000 or even 200,000 seems a more reasonable price to me.
Moral of the story is that slots aren't all that (and neither is +6 to a dump stat).