I can recommend the Radiator server from OSC ( www.open.com.au ). It is actually written in Perl which might seem a little weird but it is a top notch product at a very reasonable price.
They have excellent documentation and I have been billing from As5300s using it for about two years.
HOWEVER, this is not a billing system. You need to do a fair bit of data processing to generate CDRs from the RADIUS ACCOUNTING records. This is not a week long project unless you are or have a hot programmer.
I know that Rodopi do a Cisco compatible VoIP billing system with calling card support which is relatively affordable. A company called Telephony Experts do a relatively cheap product as well ( they merged with someone and have a new name ).
After that there are some expensive solutions.
An Israeli company makes a system that starts out ( or at least did two years ago ) at $100k. Mind CTI is the company.
Then you can go to Portal if you have at least half a million in change, but with only half a million to spend you might not interest them ( unless they have gotten hungrier with the recent industry downturn ).
Somebody else may have investigated all of this more recently than me and may have more current info to give you.
