So I have G9 Halogen bulbs in my bathroom fixtures. There are 6 total. They are 40 watt bulbs. We use the lights for about 2 hours per day. Electricity costs 13 cents per kWh. I replaced these halogens with LED bulbs that cost $19.99 for 6 on Amazon. The cost savings in electricity per year is $20. Using the 4% rule, we now have to save $500 less for retirement. So the ROI on these kind of expensive LED bulbs is 2500%. Given our usage level, and the expected lifespan of the bulbs, we may not have to replace them in our lifetime. However, if we do, it would be another $20.
Worst case scenario: if we replace them every year, we are still money ahead (G9 halogens will not outlast G9 LEDs).
TLDR: buy LED light bulbs TODAY, they are one of the easiest and best investments you can make.