Here's an energy cost comparison calculator that allows you to put in your actual fuel costs and the efficiency of your appliance, and gives you a cost per btu, so you can make better comparisons. I have discovered that currently, water heated with electricity or LP (propane) ends up costing about the same. As soon as the winter peak LP price drops a bit, the electricity is definitely more expensive. And electric prices keep going up. So we heat our water with a coil in our wood furnace, and don't worry too much about the cost.
http://www.hearth.com/econtent/index...on_calculator/