There is a reasonable debate surrounding whether an execution is more expensive than simply incarcerating for life, mostly because of the numerous appeals involved with a death sentence. I have no opinion regarding capital punishment.
Additionally, there are the questionable convictions and the legal convictions that are subsequently found to have been innocent but for which innocence is no longer considered (e.g. dobbsbob comment about Rob Will - https://news.ycombinator.com/item?id=7822471).
In many states there were mandatory appeals for death-row inmates which added to their costs. For most other sentences there were more limits on appeals which in-turn limited the life-sentence appeals costs.
There were higher housing costs for the death-row inmates, although I don't remember seeing a breakdown. I always assumed the differences were because of increased security costs. The housing costs were for entire expected sentences, not by year (while a life-sentence inmate would be expected to need housing for a longer time, the costs were still less than a death-row inmate's costs until execution).
Also, in many prisons, the life-sentence inmates could participate in work programs, with some of the related income being funneled back into the prison. At the time I was doing my research I couldn't find any work programs that allowed death-row inmates to participate. So not only were the life-sentence inmates cheaper, they could potentially work to reduce their own costs.