Lefkow came home the evening of Feb. 28, 2005, to find her husband, Michael Lefkow, 64, and her mother, Donna Humphrey, 89, shot to death in the basement. Bart Ross, an electrician whose medical malpractice suit had been dismissed by Lefkow, killed himself two weeks later and left a note confessing to the crime. DNA evidence and spent shells confirmed Ross was the killer. Lefkow said she was surprised to hear from an expert from the U.S. Marshals Service that a home security system could cost as little as $100. She said she and her husband had looked into buying a system but understood it would cost several thousand dollars. "We just didn't do it," she said. She said judges must make up their minds that "for the sake of my family, for the sake of myself, I just have to do it."
The article doesn't say how the crime was executed so there is no way to know, based on the story, whether an alarm system would have made any difference. Assuming that a '$100 alarm system' had been in place, it would likely have only benefited her family if they had been armed and ready to go once the alarm started going off. The alarm going off itself won't protect anyone from someone that is there specifically to kill you.