The one thing that you may not be aware of that will raise costs above what you may expect is that prisoner communications are generally monitored, and not just "this may be monitored for improved customer service in the future" but you know nobody is actually being paid to listen to all the calls, but actually monitored. Whether you agree with this or not, this does mean the service is going to be more expensive than a straight service normally would be. There's also significant vetting on what apps are allowed, which is also not free.

I'm not making a defense here of any particular price or practice, just giving you a partial answer to your question, in that there are costs in these services above and beyond what you would expect for a "normal" service of this type.