First, How exactly can you tell? Lags and latency have nothing to do with bandwidth usage.
Second, you're comparing large complex xbox games to an iPhone game? Are you joking? There's more stuff going in a typical console game than probably 10 iPhone games combined.
Third, since you're basing this on the xbox live, I googled it. According to an xbox magazine, average usages is around 16MB per hour. People reported anywhere from 10-50MB per hour. Of course, if people are playing games where there's a lot of voice communication going on, that's going to increase the bandwidth usage a lot. I seriously doubt there'll be voice communication on the iPhone for the sake of bandwidth and ATT wouldn't allow it over 3G, in which case, it doesn't matter.
The fact that xbox live games average 16mb per hour, now scale it down to iPhone games, it'll be far less.