Testing the math "at break even" - $15 / $2.34 = 6.4 megabits/second. 6.4 megabits/second at 30 days in gigabytes = 2,073.6 gigabytes.
So, there's enough margin for everyone to be using the 500GB without that ISP going bankrupt. (Yes, I realize that they have costs for servers, cooling, real-estate, diesel, staff, security, etc..., but this shows we're in the right ballpark with a 4x margin)
Right, at 500GB it's definitely a reasonable cost -- I think VPS providers like the parents host are much more sensible with what they advertise.
Virtual hosts like Bluehost ("UNLIMITED Domain Hosting, UNLIMITED GB Hosting Space, UNLIMITED GB File Transfer") and Dreamhost ("Disk Storage Unlimited TB + 50GB Backups, Monthly Bandwidth, Unlimited TB") however are the ones who are especially bad with their advertised offers (all for around $5-7 a month). You start using even a couple hundred GB of bandwidth a few GB of storage and they're happy to kick you off for "abusing resources".
http://drpeering.net/white-papers/Internet-Transit-Pricing-H... is a good rough estimate of Transit Costs on the internet.
For 2012, it's around $2.34/megabit/second.
Testing the math "at break even" - $15 / $2.34 = 6.4 megabits/second. 6.4 megabits/second at 30 days in gigabytes = 2,073.6 gigabytes.
So, there's enough margin for everyone to be using the 500GB without that ISP going bankrupt. (Yes, I realize that they have costs for servers, cooling, real-estate, diesel, staff, security, etc..., but this shows we're in the right ballpark with a 4x margin)