I have a kegerator that isn’t seeing much use as I don’t really get full kegs of beer anymore these days. We were buying cans of seltzer by the case and I figured I could just make my own. So I got a new 5 gallon keg for around $100 and some connectors and stuff. Got my co2 tank filled. Filled the keg with water and cranked the pressure up. After a day or two I had decently carbonated water. Pour a glass like you would a beer, add a little lemon or lime juice for flavor and boom. Seltzer. Been doing that a few years now. Between the co2 fills and the water from my tap it probably costs me $2-3 per 5 gallon “batch”. Compare that to $10-15 for a case of twenty four 12oz cans.