Practical reason? The cost of the state maintaining the person's life for however long they live. The state will need to pay for their food and medical services at a minimum, unless you plan on keeping them in utterly inhumane conditions that no first world country would ever approve of. Assuming Ask Men is correct, a 2008-2009 report by the Legislative Analyst's Office estimates that California spends $12,442 per year on each inmate. Some prisoners even get better health care than the guards, since it's viewed as the state's responsibility to care for them if they get sick or injured. $350,000 surgery and all subsequent medical bills? You get that for free as long as you're facing years in prison when it happens.
So let's say that an 18-year-old football player rapes and murders a young girl and gets sentenced to life in prison. If he lives to 72, the state will have paid $671, 868 to care for him. Should he require that $350,000 surgery at any point in his life, that's over $1 million that has been spent to keep him alive and under government care as long as he lives. With the average life expectancy in the United States and the knowledge that the prisoners will get regular meals and proper health care without ever needing to worry about the price, each prisoner who's facing life can easily end up with the state spending hundreds of thousands on them.
Regarding the cost of the death penalty that organizations like Amnesty International mention, they admit that the vast majority comes from the actual case and not the execution. Injecting someone with a cocktail of drugs or shooting him in the medulla is cheap. The expensive stuff is what has to be paid by the state for the trial and pretrial.