An excellent article about 95p bandwidth pricing.
In summary - it's a pain in the ass. Basically you pay for your peak usage - so if 36 - 38 hours out of the month you have huge bandwidth needs, then that is what you pay for, even if the rest of the month your bandwidth needs are zero.
The article explains how the bandwidth calculations SHOULD be made.
But there is a lot more to it ...
Consumers don't worry about 95p billing - they get a flat rate from their ISP and if there is too much going on then things just slow down a bit.
Producers of web content do need to worry unless they manage to find someone who will let them pay by the gigabyte. It is mind-boggling difficult to predict your bandwidth usage.
Which is why commercial ISPs love this - they can't predict your bandwidth usage either, so they dump the burden on you. The trick is this - you have to guess in advance what you bandwidth needs will be and if you go over then they charge a 2x or 3x overage fee. That could get painful.
You can get a continuous 100 megabit connection (billed using 95p) for about $3,500 a month (less if you shop around, more if you want high-quality Internap bandwidth). If you sign up for a 2 megabit continuous connection for $150, and get swamped with requests, and send out 100 megabits for 36 hours of the month, then you might get charged $10,000.00 in overage fees.