All you need to do is take your car on the ferry (or book a hire car for a minimum of 2 days on Kangaroo Island), book a minimum of 2 nights’ accommodation with SeaLink and travel by the 31st August 2011 and the $10* ferry fares are yours.
Normal fares can be found here:
http://sealink.com.au/getting_there/fares/
how much does it normally cost ???
you should add to the title that you are required to book accomodation, so it's unclear if this is a bargain or not as we don't know how much it costs…