This Philips SL*18D, an early, energy-saving fluorescent, (18W) has just given up the ghost. It was bought in 1985. I estimate that over the ensuing 25 years its use has saved, very roughly but of the right order, 700 kilowatt-hours. Taking the average energy price over the 25 years to have been 5p per kWh that means the cost saving has been about £35.
The lamp itself would have been fairly expensive in those days, probably about £10 but I suspect this has been more than offset by the number of incandescent lamps I would have had to buy over the same period..
It is very heavy weighing in at 550 grams and been in a suspended lamp-holder all its life: but the flex and anchoring seemed to stand the strain.