The registration cost for the event would be between $450-$550 depending on the dates you attend, but you may as well be attending it for free, considering some of the devices Google is known for giving away to the attendees. Back in 2010 attendees were given a choice between the Motorola Droid and the Nexus One, then in 2011 attendees got to take away a Samsung Galaxy Tab 10.1 (some of which ended up on eBay). So it is very unlikely you will be disappointed attending next year's I/O.
source - Google