A good thread for this -
Some of the cites are COVID priced, but overall it confirms the CO2 is pricey, but mods can make it more tolerable. If you buy off the shelf Sodastream syrups you’ll likely be unhappy with both taste and cost (most of them are half sugar / half artificial sweetener).
While this will be overlapping with my post in the above link, I like Sodastream and the like because my wife drinks carbonated water during warmer months, and I make my own sugar syrups, which doesn’t save me any money, but means it is exactly what I want.
And I normally just want barely enough Soda to make a weekend’s worth of Rum and Cokes.