Hi, There are a few options to choose from,
1. Open Source
Linux PC loaded with apache, mysql, php and free radius
Does the job, elbeit difficult to use.
2. Open Source + Closed Source (Radius Manager Now in Ver3.7)
Linux PC loaded with apache, mysql, php and free radius, but uses a web based GUI that is not free.
DMA softlabs has this, it's a good gui, and easy to use. I have used it for 4yrs, and it just works.
Sends clients emails with warning on nearing caps etc.
This is an all in one billing solution/radius server.
The downside is that it costs US$99
3. Mikrotik Userman running scripts for Billing Solution
I have personally tried to get this working and failed. I ended up using option 2 here.
Overall Option 1 is free, Option 3 is basically free since you already should have userman on your router, but option 2 is what I chose, because of ease of use, and even the average Joe can manage the system now