Seems to be out of stock. If you mouse over the MX518 and click on the green "Quick Look" button, you can then click the "Buy Now" button from there, however it currently says "Sorry, this item is not currently available".
I had that a couple of days back when I ordered mine. I think their web page is just a bit cr*p. Try checking back a bit later on, it worked fine for me later that day.
Expired? If you click on MX518, it just refreshes the page :(
best mouse ive ever owned, so comfy and accurate and ive owned it for what must be 4-5 years now and its still going :)
Probably lower prices as the replacement has been announced - http://www.engadget.com/2011/06/13/logitech-g400-optical-gaming-mouse-available-for-pre-order-want/