We'd know, except the two biggest companies GTL and Securus, that together make up about 70% of the market, are both owned by private equity and have no requirement to disclose.
It is known that a high percentage of fees are paid back to the prison the calls originate from [0], and that offering to return a higher percentage is one way these companies win exclusive contracts with prisons.
So color me suspicious that a profit source for prisons is being run at the most efficient and lowest possible cost to inmates...
How much is profit versus heightened cost, e.g. monitoring?