Do the prisons pay less in overhead in exchange for the higher rates?
Or is it just that the market for phone providers isn't competitive?
According to one source (below): some prisons gets a commission on each call, which ultimately would be paid for by the users/convicts. This makes sense as a reason for high prices because you have the entity (prison admin) choosing a provider with an actual incentive to not choose the lowest cost one.
The prison operator has a (joke not intended) captive market and that is exploited by contractors who often share the proceeds with the prison operator.
The tablets are also often starting to replace physical mail - inmates are being denied physical mail, instead letters and drawings being filtered, scanned, and uploaded remotely from elsewhere. Or they can write letters outbound - just have to pay for "digital stamps" - even for electronic mail. Double points for making people on the outside buy the "digital stamps" to send them inwards, too!
Every single corner is designed to extort the prisoner while making themselves look like the Good Guys for providing access to all this information and capabilities in such a safe and controlled manner and at "no cost to the taxpayer!"
A packet of Ramen in a prison store will cost several dollars. There's zero acceptable justification for this. Making a prisoner pay more for a snack isn't justice.
Also, it's not a snack, because in most states, prisoners are only required to be given two """Meals""" a day. There are very few nutritional or minimum standard requirements for these """meals""" and in many counties, there is a rule that every dollar of the budget for feeding prisoners that is not spent is given directly to the guy who sets the menu and operates the canteen.
Most prison meals in these systems look like that famous picture of a "sandwhich" from the Fyre festival.
You know, the kind of thing that would be used as an example of "Perverse incentive" in a high school economics textbook.
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.