When you say "tier," what exactly do you mean? In co-location, you'll often be on a 10Mbps or 100Mbps switch, with a bandwidth commitment in any size that the provider wishes (perhaps 1Mbps increments).
YMMV - 3200GB is the theoretical maxmium that a 10Mb/sec connection can push in one direction, if it is completely utilized 100% of the time. When getting billed at 95th percentile, average hosting traffic will utilize 180-200GB per Mb/sec. Thus, assuming you have "normal" hosting traffic, you will realize about 2000GB of transfer from a 10Mb/sec connection.
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