I think the originator of Gatorade simply looked up the formula for warm-blooded ringers solution and used that to rehydrate his football players in Florida. It is the solution that was used in your biology class to keep tissue alive. It is incredibly cheap to make if you look at the cost of the ingredients, simple sugars and some salts. If you Google on "DIY Gatorade" or "DIY sports drinks" you can come up with quite a few that are based upon diluted fruit juice or Kool Aid for flavoring. If you don't want to use fruit juice but want to increase the potassium, you can substitute the reduced sodium salts made for people who are on sodium restricted diets (my dad used to use a brand named Co-salt). They are a mixture of sodium chloride and potassium chloride. That will increase the potassium and reduce the sodium which most of us get far more than we need anyway. I remember trying unflavored Gatorade when it first came out in the early 1970s and thinking it was like drinking urine. The taste was awful.

If you don't want to bother with mixing your own, try the local dollar stores. I came across a box of 8 packages of Gatorade G2 in grape and fruit punch. It makes 20 ounces. It was one of quite a few different brands of drink mixes they had at between 10 and 15 cents per bottle.