gas heaters are cheap, easily obtainable(even 2nd hand), can be moved around and last a while when used right.Just a thought for those who are currently using electric heaters.
I don't think you really have any sort of question on your hands to be honest. Having a wetback system VS a heat pump its pretty obvious what is cheaper... The only advantage to using a heat pump in your situation is that you have the lazy and easy technology thing going. You can set it to turn on before you get up and before you get home and its the click of a button to have a somewhat instant heat source. As opposed to setting and lighting a fire and maintaining it through out the night. Another thing to consider would be having showers while the fire is running to really take advantage of the wetback...