Sure, some places charge to upgrade. Some places are metered.

100Mbps is 33TB / mo. 1Gbps is 330 TB / mo. I'm curious what they have and how they prevent saturation from a client or they just pass on networks managed by the DC.

> Many places would offer you a gigabit pipe directly.

i.e. you don't have redundancy? A gigabit is nothing these days sadly. You can get a surge in traffic (DDoS or not) and be stuck quite quickly. You always need extra capacity to deal with issues unforeseen and that adds to the cost.