Stinky,
You are correct as to your assessment of the cost of executing folks in this country. It is far more expensive to execute prisoners than to imprison them for life. Cost is indeed a consideration that supports the use of life imprisonment without the possibility of parole as an alternative to the death penalty.
Personally, I think it is a shame that in the USA we hold ourselves out as the leaders in civil rights yet we lower ourselves on this issue to the standards of China, Iran, Libya, and other countries where civil rights are nonexistent or extremely limited. I hope that some day we will evolve into a less bloodthirsty society.