It might be partly psychological. The Weasleys are poor, and they're habituated to trying to save money. Ask someone who lived through the Depression whether they would rent a nice suit for a job interview, or pay double for high-reliability smoke detectors. Or, ask yourself whether you have more than one smoke detector in each room of your house, with the batteries changed out of phase (one set replaced in summer and winter, the other in spring and fall).