Also, you'd be rolling the dice on energy costs.
No way to know what the future holds for the prices of electricity, NG, propane, gasoline, etc....
What's true today might not be true in a year.
What if after spending thousands of $$ to convert and setup your spa for propane, which as Spatech states, will likely cost you more than you think, the price of propane spikes up?
I suspect you'd break even at best on this idea.
I suspect stronger that you'd loose, but that's only my opinions.