I actually did this back in November. I averaged out what my games were actually selling for on eBay (By which I mean I only looked at the sold listings) and what I'd be willing to sell them for if I were to.
This is what I got at the time:
Nintendo DS: $106.70
GameBoy Advance: $23.68
GameBoy Color: $6
Nintendo Wii: $599.74
3DO: $104.82 (iffy as I could not find pricing on most the sample discs I have)
Nintendo GameBoy: $103.71
Atari 2600: $40.16
Odyssey2: $2.98
Intellivision: $179.61
Sega 32X: $124.83
Sega GameGear : $52.14
Virtual Boy: $154.54
Playstation 2: $26.96
Playstation 1: $114.65
Original Xbox: $156.73
Sega Saturn: $171.19
Sega Dreamcast: $208.78
CD-i: $210.96
Turbografx-16: $67.50
Nintendo GameCube: $441.19
Nintendo 64: $352.81
SNES: $736.24
Genesis: $1,139.78
NES: $1,157
Total $6,282.70
But, of course, I have gotten a lot more games since and these are very subjective numbers. I'm not gonna claim this is the real value. But it was fun to do.