How much is a life ?

A small but terrible article attracted my attention this morning. A 5-year boy shot dead his 2-year sister accidentally with a rifle in Kentucky, United States. (It was just a small article here in the morning but in the evening it became the top news on TV news program.)
They said the gun was the boy's own, given as a birthday present last year. A rifle as a birthday present for a 4-years boy??? I feel sorry for the girl but at the same time I'm really curious about how are the children's family thinking about gun at present.
I can't help thinking that a human's life is really cheap in USA.