I feel I should note that public unions will spend effort lobbying to increase workers' share of the prison costs

Workers that are union members. I don't know whether a CBT therapist would be included. If a prison suddenly starts hiring all sorts of specialists that aren't prison guards and pays it's money to them, the prison guard union might not like that.

To make the whole proposal more target, we might switch from paying the full bonus to the prison and pay half of it to the prison and the other half to the employees directly.