Whatever price the vendor you take your scrapbook charges.
From what I remember from iRemember they don't do the printing, YOU do. Even in the feature list they specifically state "and deliver the resulting files to the vendor of your choice." You can also send them to Apple "either on your own printer or by sending pages to a printing service (such as iPhoto’s book service). " and I KNOW the iPhoto book prices are on the web, they are also linked in the iPhoto/Aperture/etc software.