We don't work with numbers much, which is a good thing for a variety of reasons, so we can't get too excited about a mouse with a built-in numeric keypad. But someone must, because this is the second one we've seen lately--the first being one that looks like a miniature George Foreman Grill.
This one from Japan's Sanwa doesn't have the plastic cover but would probably suffice as long as the buttons aren't too sensitive; otherwise, it could make your work a living hell. Either way, it seems that $57 is kind of high for the double duty. Especially when there are mice out there that not only have a keypad but also serve as phones, in both slider and clamshell versions.