You need to take a look at the other limitations of that account, like the number of domains one can host and the size of MySQL space. That will, on average, reduce actual usage quite significantly. It's unclear whether they allow reseller to oversell as well. That can have a significant effect on actual usage too.
I'm not saying it's a sustainable pricing, just that the space and data transfer figures alone don't tell the whole story.
I have seen 1 TB and 10 TB etc at cheaper prices. Some do have strict SLA to protect themselves but others just do not care. I think if those guys dare to offer such ridiculous packages, they would hardly care for anything, definitely not engaging a lawyer to draft out something to protect them. It is simple maths to check. If you are running a hobby site that can be up or down and you don't care but all means. But not a reseller as you are reselling to your clients, you will be toasted by them if there are huge downtime, worse if all data disappear.
Read through our forums, there are simply too many horror stories.
Read this. http://whreviews.com/overselling-hosting.htm
This will give you an idea about overselling. And, there are actually, hundreds of posts about this matter in wht. If you do a quick search you will be able to find a lot.