Cheaper than custodial sentencing. Although a specialist mediator is needed, restorative justice is cheaper than keeping someone in prison, taxes are lower for RJ. Meaning tax money saved from doing RJ will go towards public to economically benefit society more, eg, more money towards education.
Even if RJ doesn't result in a lower rate of recidivism, then it will be more expensive to re-arrest and recharge rather than just keeping them in prison so may be more expensive if it is not successful