you could make the Winning divisible by the amount of weeks that a game has been out... like the winnings(or losses) for NSMB would be divided by 100 weeks or whatever... it is pretty pointless to pick those kinds of games anyways so this makes it so the max you could get off of a game like WiiFit would be like 80 $ or so... new releases would be a full 1.0 multiplier..
Or you could just set up a preset multiplier X how long it has been out. If you did the 90% thing like Mneth said you have to many decided each week for each game. This way you could use a simple forula
Bet/(weeks out X muliplier you create) = winnings....
End of 2009 Predictions (Set, January 1st 2009)
Wii- 72 million 3rd Year Peak, better slate of releases
360- 37 million Should trend down slightly after 3rd year peak
PS3- 29 million Sales should pick up next year, 3rd year peak and price cut
