Not sure about consoles, but with PC online gaming generally doesn't use much traffic either way. It does depend on the game though, whether it's got dedicated servers or peers to peer, and then some games are just less efficient than others.
I wondered this myself a few months back so I recorded traffic data for the PC whilst playing games:
Source engine games (mainly TF2 and L4D2) use dedicated servers and eat up around 35mb per hour down and 10mb up WITH voice comms
Borderlands (peer to peer, hosting) was about 20mb per hour download, 40mb per hour upload, with voice comms
Borderlands, not hosting, was around 50mb down, 10mb up with voice comms
Guild Wars used 30mb per hour down, 10 up
These stats were recorded with a nice little app called DU meter, however it monitors all traffic coming and going so what the games actually use could be less than that.
I did also check the usage for Killzone 2 by keeping an eye on my usage meter (also on an optus plan with uploads NOT counted), and it used between 70 - 100mb per hour down.
Anyway, my point is you probably won't have to worry about it, upload usage won't increase much unless you're hosting, and in that case the download usage will drop.
~ PSN - Trent_Steele ~ Steam
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