Here's the problem. I know that I would rather be dead than locked up forever. Actually I would take death over a lot of possible scenarios, and considering that everybody seems to react poorly to that, I assume that the vast majority of people would prefer to be alive and in prison than dead.
Economically, it's not that expensive to kill people. It's expensive to do it by the methods we currently use. Give every state an airtight room and a few giant tanks of nitrogen and you can kill quite a lot of people super cheaply.
Regarding the Lecter example.... ugh, you've pushed a personal button on the design of prisons and other secure facilities.
The human animal is not, biomechanically speaking, that dangerous. Without substantial training or weapons, it's hard for us to inflict truly awful damage on one another. We're not equipped with talons, claws, overwhelming strength, and our teeth are modest weapons at best.
The only people I believe we should put in cages are those who have demonstrated a willingness to inflict physical harm/death on others. These are dangerous animals that happen to have human shape. How do so many zoos keep tigers and lions, and all other manner of dangerous animals, with remarkably few deaths each year? By designing environments where the animal does not have an opportunity to physically interact with it's captors. With automation, and sufficient industrialization, a proper prison, by my estimation, keeps physically dangerous individuals alive, in good health, and with access to whatever minimums of stimulation necessary to keep them from going insane. All of which can be provided without direct human interaction.
Such a prison would be made of materials that humans cannot alter without substantial physical injury, poured concrete, steel bars set in poured concrete and half inch Lexan should prove more than adequate. Structures can be built in such a way that the inmate is never capable of interacting with a seam, lever, door, or any other bit that could be manipulated to come off or cause them to escape. Inmates would never interact with each other, or even with guards except remotely.