This page is to give a more in depth view of the cost per GB.
Capacity Calculations
There have been previous attempts to cost 1GB of bandwidth, and I realise this checker differs from other attempts at calculation.
The reasons why my calculation may differ from others is that:
1) I have based the costs on the price of a BT Central 622 Mbit/s L2TP Passthrough which is £1,758,693.00 pa
(which is more expensive than 4 x 155 Mbps £371,535.00 = 1,486,140) = £272,553 dearer.
Most medium and large ISP's will use the 622 Mbproduct.  Plusnet certainly do hence why I have used the 622 as a base cost.
2) I have included VAT in my costings.
The home user pays VAT on the product he purchases from the ISP therefore it makes it easier for someone to understand the end cost they have to pay.
If you are calculating the end cost to the consumer then you cant mix exVAT and VAT inc figures. You stick to one set or the other.
At the end of the day Plusnet will still have to pay/collect the VAT portion to HMC&E at some point.
Calculating the cost per GB is no mean feat and is subject to many variables therefore we have used the following method which gives an indication of the minimum cost to an ISP if their pipes were maxed out 24/7. Underutilised pipes will cost much more than this.
For the calculation I used the following formula:
var bt622cost = 1758693/12; // £ per month
var totalthroughputpm = 622 * 1.5 / 8 * 60 * 60 * 24 * 30 / 1000;
622 * 1.5 gives us the value for up and downstream which is the total throughput on the pipe
dividing by 8 gets the MBytes rather than Megabits
* 60 gets the mins
* 60 gets the hours
* 24 gets the days
* 30 gets the month
/ 1000 gives us the GB figure. (plusnet and BT Radius monitoring use 1000 rather than 1024).
var costpergb = bt622cost / totalthroughputpm;
as an additional variable we also show the downstream only figure
var gbdownstream[1] = gb[1] * 2/3;
which is the same as
622 / 8 * 60 * 60 * 24 * 30 / 1000;
Theres a very good article written by Mr Saffron from adsl guide here which explains cost per GB in more detail
The reason why his calculation comes out at 52p is simply because he has used the exVAT cost of a 155Mb pipe which is cheaper.
Aside from that all the other variables are the same.

This calculation is a cutdown of a report that I originally wrote for the PlusNet UserGroup in January 2006.
The full article can be seen here.
Note by the Author
This "report" is not just something that has been thrown together in a few hours, it’s something that I’ve been mulling over for quite a while, and has taken months to accumulate some of the data. The whole exercise for me has been enlightening in a strange way.  I started off thinking PlusNet were "tight" for not lighting new segments as and when required, but now I’ve had to face a reality check. :/
I don’t like traffic shaping  It’s always something I’ve been against  I still don’t like it and I never will, but the alternative of letting unmanaged contention kick in as and when is even worse.
 I’d much prefer a clean cap or pay a few extra £ per month... I’m also very aware that there is no way I could afford a 1:1 connection.
© Kitz  Blame me if I’ve got any of the figures wrong
Its not my intention to mislead and information stated is to the best of my knowledge.

Ex VAT calculation
Okay lets rework this using exVAT figures
var bt622cost = 1496760.00/12 = 124730 pm
var totalthroughputpm = 622 * 1.5 / 8 * 60 * 60 * 24 * 30 / 1000;
622 * 1.5 gives us the value for up and downstream which is the total throughput on the pipe
dividing by 8 gets the MBytes rather than Megabits
* 60 gets the mins
* 60 gets the hours
* 24 gets the days
* 30 gets the month
/ 1000 gives us the GB figure. (plusnet and BT Radius monitoring use 1000 rather than 1024).
= 302292
var costpergb = bt622cost / totalthroughputpm;
(124730/302292) = 0.4126 per throughput GB (+ VAT 0.072205 = .4848)
Cost per GB downstream =
.4126 x 1.5 = 0.6189

So now we have the exVAT figures
being .41p and .62p respectively.
Now comes along Joe Bloggs and he says how much will it cost for me to download extra GBs?
The ISP will have to charge VAT cause its a rated service  all be it that portion goes to C+E
0.41p x 17.5% = .4848
0.62p x 17.5% = .7272
Now when I worked it the other way round.. it came to
...... 49p and 73p
