Given the Gamecube and Xbox were only discontinued in the past 5? years they are still readily available. The Amiga is significantly older and there are probably less working examples now.
I guess that the Gamecube and Xbox will rise in value when they are over 10-15 years old as they become rarer and more break down leaving less working examples.
I also agree with Coze, the bond is stronger with computers such as the Amiga.