Cannon Spike is one game that has never made it to another system in any form, which I would assume is playing heavily into its price. I bought mine years ago from EB Games for less than $30. I was just playing it the other night with a friend of mine that has never played a Dreamcast (blasphemy I know!), and she was beside herself at its niche/fan service awesomeness.
My guess is we'll be seeing a lot of games yet to make it to other platforms (like Zero Gunner 2, also one of my favorite shooters) go up in price.