About 18CHF in Migros

Basically, if you go to a dealer who takes back the empty cartridges, you only pay for the filling. I don't know the exact price, but you have to ask such a dealer, or Sodastream on their homepage, where everything is described, or you can also contact them and ask for information.

The empty cylinders cannot be returned to Digitec Galaxus. You have to send them directly to Sodastream.
You can only buy new ones at Digitec Galaxus.