i literally just finished recharging the bottles on my two bigger tanks, in which i use two bottles each.
in a two liter soda bottle, or a 1.75 liter juice bottle, i use a recipe of 1.75 cups of regular white sugar, 1/2 teaspoon of bakers yeast, and 1 teaspoon of baking soda.
first i take a cup, and fill it with about a quarter cup of water, add a pinch of sugar, and then add my half teaspoon of yeast. i stir it with a fork, and set a timer for 10 minutes, stirring the yeast every 2-3 minutes. in the meantime, i measure out my sugar and baking soda, and add it to the bottle, then add hot water to fill the bottle about 2/3 full. then i shake it up to dissolve the sugar. after the ten minutes is up, i add the yeast concoction to the bottle as well, and shake for 2-3 seconds, then cap it and it's ready for the tank. in about 2 hours, it should be producing CO2.
the reasoning for adding the baking soda is to prolong the life of the yeast. as the yeast produces CO2, the water in the yeast bottle becomes very acidic, eventually killing the yeast. by adding the baking soda, it kind of counteracts the acidity, and helps keep the water neutral, allowing the yeast to survive longer.
once you do the work to construct your CO2 bottles, with the air hose and caulking, there is very minimal work involved in maintaining it. as long as your bottles are airtight, it should work well for a long time. i run diy CO2 bottles on six different tanks, amounting to a total of 8 bottles, and probably do less than an hour per month of work to recharge the bottles with new batches. and for those six tanks, i'd say it costs me less than $10 a month, which would be for the sugar and yeast. to construct my 8 bottles, i originally maybe used $15 dollars worth of supplies.