Cartridges only cost between one to five dollars a round, cost saving there.
Far less than housing, feeding, clothing, educating, medicating inmates over the course of a life sentence.
The problem is it takes about 10 years to execute someone because of all the appeals required by the Supreme Court rulings. It costs considerable less to house and feed someone for 40 years.
http://www.deathpenaltyinfo.org/costs-death-penalty-costs-texas-outweigh-life-imprisonment