Well, the estimated price ranges are not final (and I see some swings in both directions), but that aside, I tend to agree that 20% (items less than $10,000, I believe) seems like a big chunk. In that regard, one has to consider what some of these cost on abe, and in that, some of the ranges on the lower-cost items might still suffer due to the 20% buyer's premium. But PBA ain't no ebay - they approach things like a proper auction house (marketing, proper photos, descriptions that won't leave you wondering what you're getting, etc.), and in that light, the cut is not out of line with the auction market in the U.S.
Edit: unless I'm missing something - does the seller get full auction price and only the premium goes to PBA? Or does the seller also relenquish a % to PBA?