A recent post on the official Android Developers blog tells us that the sells for $529 as part of the Development Phones handsets. Previous developer phones include the G1 and the myTouch 3G/Google Ion.
Although the phone ships with Android 2.1 under the hood, it will grab the Froyo update shortly after powering on. If you are an Android developer and want to get your hands on the Nexus One, this might be your last option. There is no indication as to how many of these will be available or for how long.