The Chatpad add-on has been priced and dated according to Xbox.com. When looking at the details for the Xbox 360 Messenger Kit (which is the Chatpad along with a headset) it lists the MSRP at $29.99 USD which is a pretty reasonable price. The release date is not on the product page but if you search for it, as Xbox 360 Fanboy pointed out, the results show the date.

Not sure how long that info has been there or if it found it’s way there just ahead of the press briefing being held tonight. We’ll see if it’s a topic they mention.