It usually sells for about $25 bucks. I know that sounds like a lot for a GC game, but all things considered, that isn't too bad. The fact that it still cost that much shows that it is still a very desirable game.
Of course, I believe you are from the UK? So it may very well be a different story there.
As I said before, Twin Snakes was more for those who had never gotten around to playing the first game before.
I think it is kind of funny, is how its name and cover art provide spoilers for the actually game. It would be like Empire Strikes Back having been titled, Star Wars Episode V: Father Vs. Son and having Luke dueling Vader on the poster.