Basically for a new game the RRP is $69.99, so anyone smart will pay around $50-60 for that while it's new and in a sale period. The two games you mentioned I would think would fall down to the $30 price range sine they are no longer really new/top range titles. As for ebay like anything people expect to find things cheaper there and I have noticed with DS games they generally go for way below the RRP.
22/07/2008
The World Ends With You
In Your Base Killing Your Doodz!