You could easily extend that argument to say "Why do we house, clothe, feed, and care for any criminal?" If you're going to kill a murderer because it'd be too expensive to keep them alive, then why not kill rapists too? Gang members? Robbers? Thieves? Where does one draw the line? Because by that logic, the most cost-effective way to treat criminals is to kill them instantly, regardless of the crime.

But of course, the criminal justice system exists for a reason. Jail time acts as a deterrent to harmful behavior, with the length of punishment proportional to the crime. And if someone is truly an irredeemably evil mass murderer, we can simply lock them up for life. That, in my opinion, is worth what it costs in taxes, since the alternative is our society dissolving into anarchy.

It's also worth mentioning that it actually costs more to execute someone in the United States than to house them for life, because of how slow and clumsy the process is. So even if one would put a price tag on human life, it's still the cost-effective option to do away with capital punishment.