I was at costco a few days ago and they were selling LED bulbs at a discount. A $14.99 package of 3 bulbs (60-watt equivalent, uses 9.5 watts) were on sale for $5.99, so $2/bulb. I already use almost all cfl in my house, and they are already pretty energy efficient so it will take between 3,000-4,000 hours for the energy savings to equal the cost of the bulb, so I don't know if it makes sense to replace a cfl that is still working, but it certainly makes sense to replace an incandescent that is still working. The bulbs are also supposed to have 25,000 hour life, so it totally makes sense to replace a cfl as they burn out too.