IMO if the book is printed with heavy stock paper and the printing quality is there $99 is not a large price tag for a hardcover book that's almost 500 pages. I'm curious to see how well the material, i.e. uniforms, equipment, weapons are covered. Another thing to consider is what other titles are out there that covers the subject in a similar fashion for less money? Schiffer Publications wants $70 for a book that is barely over 200 pages.
$100 for an illustrated reference book of this size seems reasonable to me. The Rouselotte(?) Book of French napoleonic uniforms lists at $120 (I scooped mine for $60).
If this was my area of interest I would definitely buy it... it looks to be very well done.