I was always under the impression that the only time an electric heater will save you money is if your propane heater isn't divided into zones and you only want to heat one or two rooms. In other words, if you only need to heat the bedroom over night, instead of heating the whole house with the propane heater, just put a little electric space heater in the bedroom. For the most part though, gas heat is more efficient, and thus usually more cost effective, than electric heat. Of course it will vary from place to place, especialy if you have to factor in delivery costs.
depending on how old the furnace is, you should be ably to find the efficiency rating. same goes for an electric heater. look at your utility bill and find out what you are paying for electricity and do the math.
One gallon of propane contains about 91,690 BTU's. multiply that by your furnaces efficiency rating, and you have the number of BTU's you use per gallon. times the number of gallons, and you have the total BTU's used to heat your home. (with a little effort you could subtract the aproximate amount used to heat your water.) A new electric heater will tell you how many watts per BTU, times your cost per watt, and you have the cost.
long winded and discombobulated, I know
but hopefuly that helps some.