Just because the lawyers, judges, clerks, etc., are already on staff and paid does not make the cost of trial negligible. Anything those people do in conjunction with a trial is a cost. Plus, if anything extra had to be done—expert witnesses, etc., that drives the price higher.
Murder trials, it seems, cost anywhere from close to a hundred thousand up to millions.
Incarceration costs about 47,000 per year in CA (the only state I could find data).
This guy was a net cost saving to taxpayers, even with the ICU costs.
No, it makes the incremental cost negligible.