I think they'll have to start with 20Gb or 30Gb caps as entry-level.
This will be more than enough, but they'll have to revise this at regular intervals.
Once you've reached your cap, then you can buy extra bandwidth.
A more realistic way to look at things will be :
Lite email and surf package : 5Gb cap (for grannies and ppl who just check their email seldom and who want to write to their children and grandchildren once in a while)
Email and surf package : 10Gb cap - for ppl who use email and a bit of surfing on a frequent basis
Home package : 20 to 30Gb cap (allow a bit of downloading, as well as gaming)
Advanced home package A : 30 to 60Gb cap (power users who download every pr0n site)
Advanced home package B : Same as the 'A' package, but with static IP.
BusinessLite + static IP : 60 to 100Gb - small businesses.
Business + static IP : 100 to 200Gb - medium businesses.
BusinessMax + static IP : 200Gb to uncapped - large businesses.
Just a thought from my side.
You can also top-up should you've reached your threshold.
IMO they can throttle slightly during office hours to provide a good, responsive service for business users during office hours, and a good service after hours for gamers.