As an update I believe that the 656 page edition does exist, but I am guessing that it might have been a limited run. It seems like most of the people who commented on it elsewhere on the Internet are from the UK, so maybe it was only made available there.
They changed the page count under the product details of this book to 512 pages and kept the mention of The Hour of the Dragon in the product description, and the higher price. My best guess is that you will get the 512 page edition with the nine stories if you order from this page (which does not include The Hour of the Dragon, see my original review below).
I still really like the printing regardless of the page count, but I think it can still be picked up at other places for far less than the amount listed above.
Along with the availability of the Del Rey three part edition (The Coming of Conan the Cimmerian: The Original Adventures of the Greatest Sword and Sorcery Hero of All Time!, The Bloody Crown of Conan (Conan of Cimmeria, Book 2), and The Conquering Sword of Conan (Conan of Cimmeria, Book 3)) which apparently are all of Howard's Conan, including fragments and a draft, there is this Prion Unabridged (but incomplete) edition with 656 pages, and a Prion Unabridged edition with 512 pages and the same cover.
The Prion Unabridged edition I bought elsewhere, discount priced to under $10 at the time, features The Hyborian Age and 9 stories with a 512 page count: Shadows in the Moonlight, Queen of the Black Coast, The Devil in Iron, The People of the Black Circle, A Witch Shall Be Born, Jewels of Gwahlur, Beyond the Black River, Shadows in Zamboula, and Red Nails.
This Prion Unabridged edition appears to be the one that people on the Internet are talking about. It has a 656 page count, and apparently contains (as the Amazon product description lists above) the Phoenix on the Sword and The Hour of the Dragon, and more (18 total counting The Hyborian Age?).
In my opinion, when it comes to presentation, the Prion editions win over the Del Rey. It's subtle, but the slightly larger text compared to the Del Rey and the higher quality, thicker paper is very easy on the eyes. The cover is nice too, kind of like a tougher but slightly bendable paper with a sturdy feel that I guess is supposed to resemble leather by sight, and does a good job of carrying the illusion.
After reading The Hyborian Age, and then the 9 stories in the order listed, I felt saddened that all we have are these 9 stories, until I realized that there are plenty more, so I bought The Coming of Conan the Cimmerian Del Rey edition.
The Del Rey Coming of Conan the Cimmerian has quite a few stories the 9 story Prion Unabridged edition doesn't have, like The Vale of the Lost Women, and the God in the Bowl. According to the Internet, those two stories are also apparently missing from this 656 page Prion Unabridged edition and are found in the Del Rey The Coming of Conan - but at least this 656 page Prion Unabridged edition has the Hour of the Dragon, which means after buying it you can skip The Bloody Crown of Conan, because you will have all three stories they list - check out the Amazon preview.
I also liked the illustrations of the Prion edition. I may be really in a minority of opinion here, but I liked not getting an illustration until after Howard weaves his magic of description... and I liked how the illustration is merely there to illuminate a key scene (there are very few illustrations in the Prion book, maybe about one per story), which is preferable to the over illustrated IMO Del Rey where you often start the story or chapter with some kind of spoiling illustration, check out Rogues in the House for a fine example of that.
However, given the age of the Conan stories, this may be less of a fault for older readers of Howard who might appreciate it and view the artwork as a bonus that adds value to their purchase. I am in no way trying to batter down illustrators here, just realized it might seem that way - The Savage Sword books make use of illustration in wonderful ways IMO.
So to end this review (my first) after realizing that I probably rambled a bit, I would say if you are in a bookstore, a new Robert E. Howard reader, low on cash, and see the 512 page Prion edition of this priced below $10, buy it. It's a real joy and a great bargain.
If you can afford spending upwards of $15 more for a 656 page longer edition of the book with the same cover (argh) try for that, and then pick up The Coming of Conan the Cimmerian Del Rey and The Conquering Sword of Conan Del Rey for a complete Conan. Otherwise, for completists, if you do happen to pick up the 512 page Prion edition you also have to pick up all three Del Reys to get the remaining stories.
I know it works out as slightly more buying the 656 page Prion edition and the Del Rey books 1 and 3, but again the 656 page Prion edition would be worth it for new readers due to what is in my opinion a higher quality presentation that allows for a more immersive experience.