Right now, the first nine Young Wizards books are available on ebooksdirect at half off. That's $20 for nine books.

To provide a few more details, these are the New Millenium Edition releases, which rewrote the first nine books with 2008 as the start of the timeline and edited them to make the progress of time actually logical and consistent (the original Young Wizards timeline is kind of notoriously a mess, in that characters ages, birth dates, and the actual years do not match up at all because each book was set in its year of release...), with other updates and adjustments (content has been edited, and also I think there's some expanded scenes?), and is the official Young Wizards timeline from Book 10 onwards.

Also, ebooksdirect is a thing I just found out about! It's Diane Duane and Peter Morewood (her husband)'s official e-book store, where they sell ebooks of every book of theirs that isn't somehow locked down by publisher obligations (unfortunately this does mean a lot of books are missing...). It's not just Young Wizards that's on sale: everything on the site is 50% off right now. And all of it, absolutely all of it, is DRM-free: you are not restricted to one platform, one e-reader, or anything of the kind, and nobody can take away your copies of your books that you bought because they decided they wanted to pull them. Because Diane Duane is like that.

So. Y'know. Thought that might be of interest.