I use Everclear in my Super Gram Weenie. That actually works better than anything else I've found. I can boil 12 ounces of 40 F water on 1/2 ounce (15 ml) of Everclear at room temperature.
So, a 750 ml bottle would give you 50 12-ounce boils, or 75 cups boiled per bottle.
I can get a bottle for $14, which will cost $0.19 per cup of water boiled.
Now, granted this is in a laboratory setting (my kitchen), which will vary greatly in the field. ( But, I do get get a solid rolling boil for about 45 seconds, which, from my experience, will give me at least very very hot water even on cold days, which is all I ever need)