The average website - and I'd say this definitely can be extended to an affiliate type website - takes up roughly 50-100MB of space and 1-5GB of transfer. Most cases being at about 1-2 GB of transfer. In example, I have a 1600 member forum that just reached over the 5GB plateau for the first time last month.
I like to think of a premium host as mission critical. If every second of downtime costs you money, then you want a premium host. (Now, I don't mean every second of downtime annoys you or a complaint or two makes it to your inbox.) Obviously, a shared environment doesn't go down all the time otherwise the service would be useless, but in a shared environment you are sharing services with other users. These thousands of GBs hosts have put anywhere from 100+ people all thinking they have those 300GB of space (consider the size of the largest hd's that run 1TB) and 3000 GB of transfer (3000x100=300,000GB of transfer). Do you really think there's a profit to be made if you are using anywhere from 100k to 300k of transfer a month for a $595 profit (100x5.95/mo)? Even at $0.05/GB cost that comes to $15k/month in bandwidth costs for them.
They're hedging their bets that 1.) You'll never use that 300GB/3000GB (and you won't) and 2.) That even if you start to, CPU/RAM usage will cause you to be forced to upgrade to a server long before then.
There's a reason you see hosts selling dedicated servers for 10x the cost but at 1/2, 1/3, or less of the resources available. Overselling...which is what this process is called...is not necessarily a bad thing because it reduces costs on you in the end. However, it becomes a bad thing because when taken to the extreme it becomes highly misleading and ultimately it can be argued that they're marketing a lie.
