I feel that common games on the 360 will stay cheaper longer than popular games on older consoles. In my opinion those that have nostalgia attached to the 360/PS3 are more content to own their game’s digitally, so supply could hold out for longer.
In addition I wonder with all these HD remasters/ releases if people will value the collectability of the originals as much. I kind of see this with the Atari 2600, where inferior ports of arcade games on the system have near 0 value (though they so in my heart). Though this could just be a product of how rudimentary games from that time are.