LED light bulbs are incredible, in my opinion the same as or even better than, the old school incandescent ones.I got mine at a special sale (I think it was at a Home Depot) a while back, they were something like $8 or $10 each.

Bellingham's housing market is SMOKING STEAMING INCANDESCENT ABOUT TO ACHIEVE NUCLEAR FUSION HOT with housing prices jumping 11%...LAST MONTH.  

You could maybe cut that cost by making one of those lights a motion detector, but I think that a full time florescent bulb is going to be cheaper to run than turning an incandescent or mercury vapor off and on.I've got a dusk to dawn florescent security light that is 14 watts.

LED/CFL and even Halogen light bulbs are more efficient than incandescent lighting.

If you want to replace a regular one then just remove the old/regular one and install the new dimmer one.Nut remember that you can't use a dimmer switch with fluorescent bulbs, they only work with some LED bulbs and incandescent.  

One other thing you may want to consider is to switch all appliances to energy efficient models, switch light bulbs to CFL or eco-incandescent bulbs. 

  :) If they are still running incandescent lamps, you can buy a little headroom quickly by switching to LED. 

I sent the tenants an email that I wanted to come over for a regular maintenance visit, and they mentioned the burned-out bulb; I brought another one and swapped it while I was already there for other things.The bulbs that burn out the most on me are the incandescent globes over the bathroom mirror; I like the look enough that I haven't switched these to LED. 

Was it an led, fluorescent, or old fashioned 100w incandescent?

If the two "good" lights come on and the "bad" light stays off, then you have some kind of ground.A better (IMHO) test is to use an incandescent lamp in a socket with wires on it.