All energy saving bulbs are BOGOF, which means the 6 year life bulbs are in the range £0.69 to £0.84. I've replaced all the bulbs in my house for around a tenner, and feel virtuous regarding the environment ,and saving on energy bills too.
Tesco online is out of stock, but the three Tescos near me are full of them. Go on save the planet!
Here's the nerdy bit. If you used 10 (60W) bulbs for 2 hours per day, 365 days a year, you'd have used 438 kW of electricity. At £0.12 per kWh, this is about £52 per annum of your electricty bill on lighting alone.
Energy saving bulbs use 80% less juice, therefore you'd save over £40 per annum. At a £8 to purchase the bulbs cost the payback is around 2 months. So after 2 months, not only are you saving money, but your keeping East Anglia from submerging.