You can get an approximation by comparing your local utility costs. An electric WH is nearly 100% efficient. A typical gas WH is in the 0.6 range, so you'd need more energy to achieve the same amount of heated water. You can look up a BTU-Kw conversion rate, then find the BTU/gallon of your propane (note not all mixes are the same so you may want to ask your distributor). Then figure it out. You'll be close. Usually, gas is cheaper, but if you lived somewhere where the electricity is super cheap like my father where he's paying about 3-cents per KwH, gas is likely cheaper.