You get what you pay for.

3 TB of bandwidth for $9 a month is a joke. What they're doing is putting you on a server with probably 300 other customers and hoping that none of them will actually use anything close to what they're paying for, because if they did (a) the host most certainly doesn't have the bandwidth to support it and (b) the server itself probably wouldn't be able to handle the throughput.

If you really have a half million *visitors* a month and not *hits*, then you should be on a dedicated server. How much bandwidth you need is a function of what type of content you have, how much of it, and how many people are viewing it. Expect to pay a rock bottom price of about $100/month for a self-managed server and 1500-2000 gigs from one of the large providers with not-so-great support, and $175-400/month for a fully managed server.

Just to give you an idea, the absolute cheapest bottom feeder bandwidth from off-brand transit providers is in the range of $15-20/Mbit in volume. So to be able to provide you with that level of transit, they would be paying a minimum of $200/month for the 3000 gigs of bandwidth they are offering to sell you for $10.