If you have a homebrewing store nearby, chances are they have the Rainbow brand soda concentrates for $4-5. Mix them with tap water and sugar (or honey or whatever sweetener you prefer) according to directions, add 1/8 tsp. of champagne or ale yeast (natural carbonation), bottle it in a 2L plastic bottle and leave it sit for 2-3 days until the bottle gets firm with carbonation. Let it mellow in the fridge for a week or so and then enjoy some of the best soda you've ever had. The root beer, sarsparilla and orange are my favorites. Be sure to decant the bottle to keep any sediment in the bottom. The yeast has no taste, but is rather cloudy. Don't use bread yeast!
With thousands of sodas available for the choosing, why drink just one or two?