Simple, since human errors will result innocent people being sentenced. Then abolish prison altogether. what right does the government have in keeping possible innocent people in jail?
If there was any possible doubt, the death penalty shouldn't be on the table. If we knew there was iron-clad evidence that that person did the crime, then your argument doesn't stand. Just because there is doubt, it doesn't mean you just ban the punishment. Otherwise, by the same logic, if an old person commits a crime, you shouldn't jail that person, because there is a possible chance for him/her to be innocent. And when you find out, he may have died during his prison time. Death penalty is more expensive due to 1. other trials are treated too lightly about being correct. Not because death penalty causes it to be more expensive, but other trials are too cheap. On top of that, there are people like you who support super human rights that makes the death penalty that much more expensive. and punishment never was meant simply to reduce crime, but gives a sense of responsibility for your actions, bring peace to the victims and their families, and more.
Also I thought it's a debate about should there be a death penalty or not, but both parties were talking something, though related else. she asked "so you think there should be more or less death penalty?" That's a stupid question set up as a trap. How many death penalty dealt out should depend how many people deserve them, ie if no one commits a crime that would need death penalty as a death penalty, there shouldn't be any. By asking if there should be more or less is basically reducing legal proceeding into forms to fit in, but not case by case basis.