Build a terraced end house with an additional unit (GÜ) on your own

Besides, the CO2 bottles for the Sodastream are not that cheap at all...

I don't think so at all. We have had it for many years and wouldn't want to miss it anymore. Much cheaper and no lugging around.

Somewhere once saw a bill that the liter costs around 30 cents.

Even if it would cost the same as the soda maker or even a bit more - not having to carry it anymore is definitely worth it. Especially since here we have tap water of mineral water quality. I drink almost only water, so that adds up.

Somewhere I saw a calculation that the liter costs about 30 cents.

A refill costs ~8€, according to advertising that makes 60l. I would estimate that it’s rather 45-50 (depending on "fizziness").
You come out well under 20 cents this way.
But it’s also the gain in comfort and saved storage space that have stopped us from buying water in crates for years. Especially since the necessary transport routes seem suspicious to me anyway, when we have such good tap water.

We get about 2-3 months out of one cartridge for four people, and I think it costs about 7.50€. On vacation, we always notice how much water we have to carry. However, we usually carbonate at Medium or even a bit less, definitely not on Active. So we carbonate a lot of water with one cartridge.