What are the costs of violent crime in general? Take away the guns and violent crime increases without the threat of being killed.
Wait, so you're suggesting that if guns were to go away tomorrow, that those 10,000 people who would have died via guns would still die via knife, fist, feet, tv set?
No. But if there were no more guns, the overall violent crime rate is sure to go up. Aggravated robbery, rape, battery, attempted murder, etc. would increase knowing that the victim or home would be gun-free. It's sure to affect a larger portion of society than is currently affected by gun murders.
Because deterrence has decreased crime in any measurable way? Violent crime goes up and down due to the make-up of our social institutions not the presence of guns in households.
Spend 10 minutes looking into why violent crime has decreased in the United States over the past 15 years or so and you'll come up with a list including all or most of the following.
1) High incarceration rate (world's highest actually)
2) Tougher sentencing (3 strikes, etc)
3) End of the crack cocaine epidemic
4) Better technology to secure firearms and increased safety features
5) Demographics (population is aging fewer young people)
6) Improved education
7) Decrease in lead exposure and poisoning in urban areas
8) Steven Levitt even thinks abortions have contributed greatly. The thought that terminations of unwanted births that would have occurred in unhealthy environments has helped decrease violent crime.