Sure - I don't think I would buy a PIR (or timeswitch) just to avoid the electricity cost of leaving a 5W light on permanently - as you imply, a 5W light on 24/7 is only going to cost about £7 per year to run and, quite apart from the cost of the PIR/timer, those devices are, as you say, bound to use some power - maybe comparable with that used by the light!If the PIR has failed not sure if really worth replacing? At 5W and not sure what the PIR takes it would need to be on for a very long time to equal the cost of a PIR. Thought about simple plug in timer but again how much power does the timer use?
However, there are plenty of reasons other than financial for not wanting a light to be on 24/7, particularly outdoors at night.
Kind Regards, John