Heritage Auctions will auction the Frank Miller cover art for Batman: The Dark Knight Returns August 1-3.  The is the first cover from the series to be auctioned.  A splash page from the interior of #3 set a record price for American art back in 2011 (see "Miller Page Goes for $448,125").  This piece seems destined to exceed that, if only because it’s a cover and because of the significance of the series. 

The same auction will also include a CGC 9.2 copy of Batman #1, the highest grade copy known of the book.