I have been making this recipe based on Gatorade Recipe : Final recipe and really like it:

1 Kool-Aid packet
tsp Morton Lite Salt mixture (sodium chloride and potassium chloride)
1 cup sugar
1 gallon of water

I prefer to use just one Kool-Aid packet because it is easier on the stomach and has a nice mild flavor. The packets are 20 cents each and there are lots of different flavors.

About 200 calories per quart

I calculate the cost at 48 cents per gallon or 12 cents per quart. It is amazing to think what people are paying for flavored water.

One limitation: the mixture is not shelf-stable like commercially bottled products. In other words, if you leave it in a hot vehicle for more than a day or so it will start to turn into alcoholic "hooch". However it can be mixed on the spot from plain water, obviously.