HI Millwall, Thanks for your question.
To have the capacity of 500 listeners at a bitrate of 128 or 64 kbps will not cost any extra but the actualy bandwidth used depends on how many listeners you have connected. Its unlikely you will have the full 500 listeners for the full 2 hours so working it out based on this will not be an accurate cost.
Streaming voice (talk) at 64 kbps or 128 kbps is probably too higher bitrate to use and mainly for music so you could even have 32 kbps as your bitrate which would save you even more money without much loss in quality.
So as an example i will use 128 kbps for 1 hour with 100 listeners as a base calculation :
Bandwidth Calculation Results :
128 kbps with 100 listeners for 1 Hour (3600 seconds).
5.49 GB
5625.00 MB
Price : £2.75
This price will is based on buying 10 GB of bandwidth for £5 so if you were to buy more bandwidth you would recieve up to a 33% discount (when 120 GB (£40) is purchased).
You can also obiously multiple this by 2,3,4 or 5 for 200,300,400,500 listeners. Or divide it by 2 to make it 64 kbps instead of 128 kbps.
We have a bandwidth calculator here if you would like to do you own calculations :
Internet Radio Bandwidth Calculator
Also bear in mind this is an estimate as the listeners connected is very likely to fluctuate during your show.
Hope this helps ?