I have one and it is very similar to the grip for the M. It attaches to the camera via the tripod mount and has an additional strap lug as well. They are very well made and definitely add to the secure handling of the camera. My middle finger always found itself on the lens release lock while holding the camera. Needless to say I was always afraid of inadvertantly releasing the lens. Now that I have the grip, I now longer have that problem.
Yes, they can be quite expensive and hard to find. I did get mine on ebay and was lucky to pay under a $100. I guess no one was watching at the time! They can go for over $200 easily.