Yes, it's only using 20w, compared to a 100w, so it only costs a fifth to run.
Basically 1 unit of electric is a 1000 watts used for an hour, so our 20w bulb will take 50 hours to use 1000 watts, whereas the 100w bulb will only take 10 hours.
The new LED bulb (8.5w) I've just purchased for £13 would take 95 hours to use 1000w, this replaced a 25w CFL, which replaced a 100w incandescent bulb. I'm very pleased with the LED, it is instant on and is nice and bright.
Now a 100w normal bulb probably costs about 85p, now my LED bulb costs £12.15 more, to use £12.15 worth of electric I'd need to have my 100w bulb on for 1050 hours or 43 days, or 2 hours per day for around 1.43 years.
So over the lifetime (it has a 5 year guarantee) of the bulb I will eventually start saving money, and save the price of the bulb many times over.