Through October 31st, Scholastic Store is offering teachers only, 50 Book Collections for just $50 (regularly up to $350)! They have both informational text and paperback collections for grades K-2, 3-4 or 5-6, and all are highly rated.
Note that shipping is $4.50, which means you can get 50 books for your students delivered for just $1.09 per book (titles are subject to change and availability).