So how much reprinted content is in Xanathar's Guide?
~4-5 pages of subclasses
~10-14 pages of spells
37 out of the 77 wizard spells in XG are reprinted from Princes of the Apocalypse (the same spells also appear in the Elemental Evil Player's Companion). Every wizard spell from POTA is being reprinted and, based on that, I think we can expect that every other classes' spells will be printed in full as well. Since 3-5 spells tend to fit on a single page, we can expect ~10-14 pages' worth of reprinted spells.
When you subtract 14-19 pages of reprinted material and 20 pages of appendices that many people find useless, Xanathar's Guide is a mere ~155 pages. Ouch.
Then there's the fact that almost all of the content is taken from UA. Frankly, I don't expect the DM tools to differ all that much from their UA versions, so I feel like I already own a large portion of this book.