In this recording from the Maker Faire, Arduino's Massimo Banzi tells that the basic robot will cost about $159 EDIT:$259 (see minute 5:30).
EDIT May 19: This article claims that the price is $275. Also, in the video linked there, Massimo seems to say $250-259 (but it is quite difficult to understand in this one, see minute 03:40).
Listening to the first video again, it seems to be indeed $259.
EDIT May 26: Sells for $275 at Makershed.