So about 21 cents per ounce for store-bought evaporated milk and about 6 cents per ounce once you've boiled whole milk down for homemade, right? Hard to argue with those numbers….

But even if wind or the sun powers your cooking device, you'd have to figure in some fraction of the high cost of installing that system if you want to fairly compare the relative costs of store-bought vs. homemade evaporated milk. If your stove is powered by conventionally produced electricity or natural gas or propane, you'd have to do some complex math to add your energy costs into each ounce of milk you evaporate. And evaporating any substantial amount of milk takes a while. The companies that commercially evaporate milk do so with heat, but also under a vacuum, speeding the process and probably doing it more efficiently than you can yourself.

Also, to be fair, you'd have to figure in the cost of cleaning the pan, including heating the water and possibly running a dishwasher. If the milk scorches, as will happen if you have the heat too high, the pan is quintuply hard to clean (requiring more costly hot water, detergent, and dishwashing).

If you drive to the store just to pick up a can or two of evaporated milk, of course the unit cost soars, but if you purchase it on a regular shopping trip, the transportation cost can be negligible. And – unless you have a cow of your own (which would involve a variety of other costs) – you're driving to the store anyway to get the milk you intend to evaporate.

Finally, having pasteurized evaporated milk in a can makes it always available. If you make it yourself, you'll have to use it within a certain number of days. If you wind up throwing some out because it spoils before you get to use it, you can kiss all your cost savings good-bye. Recycling the store-bought milk's can is a cost variable we just can't begin to contemplate.

It is quite possible, though, that you're right – that an economist could calculate all the fixed and variable costs and that you would come out ahead financially if you evaporated your own milk. But it would certainly not be the three-to-one ratio suggested by the retail prices you started out with.

To most people the savings would not justify the extra work. But that's the great thing about cooking: you can do what suits you, your budget, the amount of time you have available, the ingredients and equipment available, the people you are cooking for, etc., etc. Those variables are also impossible to quantify, so do what feels right for you. If that includes evaporating your own milk, enjoy it!