I got the stats from an ethics textbbok, and you might be able to find them on the Amnesty International website. The extra cost of the death penalty comes from all the extra legal proceedings and stuff to ensure that the person being executed is actually guilty.
Also, inmates can work in the prison to pay for those necessities and thus reduce the cost of their upkeep. I once heard that at one time in Texas, the cost of keeping someone in jail was under $500 per year due to all the work that the inmates did.