Slop.

750 mL of fruit syrup costs around $6. In a 1:8 dilution that's 6 liters. That doesn't beat $1 for a 2L of store-brand soda, but it's still rather cheap.

Now if you really want something fizzy and customized, you get a 1-gallon glass jar of pickles, eat the pickles, save the jar. From there, about ¾ pound of sugar ($0.45), 5 regular black tea bags ($0.30), 2 tablespoonfuls of that fruit syrup ($0.24 but you can skip this), a generous dash of vinegar ($0.05), and a kombucha starter. Brew the tea ($0.01 in electricity), fill the rest up with water, let it sit for a month, and then you have 3.5 L of a fancy, low-sugar, carbonated, probiotic drink that cost you about a dollar in supplies.

The soda syrup is expensive as fuck (relative to water, smoothies, etc.) basically no matter what unless there's one people can make themselves. Even getting no-name syrup in bulk from restaurant distributors is barely cost-effective compared to getting 32 packs of soda for 15-17$ from Costco.

wtf, does America not have squash/cordial? I get 2 litres of Vimto concentrate for like £3, which makes about 10-20 litres of soda depending on how strong you like it.

The ingredients are too exotic and expensive compared to making your own alcohol or just carbonated water or fruit juice or something.

Well, they're not exotic, per se, they're just used on massive quantities in complex supply chains. It's all about scale.

Yeah, I should have said zero sugar flavor packets, coffee syrup sweatener, or juice. But the neat thing about owning your own carbonation tank is you can experiment with different options. Your results may vary!