Water filtration and osmosis
The city's hard water feeds a whole market of filtration: carbon filters, softeners and reverse-osmosis under the sink. Each level solves a different problem and costs a different order of magnitude.
The honest baseline
Barcelona's tap water is potable and controlled; filtration is about taste, limescale and specific building-pipe concerns, not safety. Anyone selling fear first is selling the wrong unit.
Carbon vs. softener vs. osmosis
Carbon improves taste cheaply; softeners protect the installation from scale; osmosis demineralizes almost completely and wastes some water doing it. Match the machine to the actual complaint.
Maintenance is the real cost
Filters and membranes are consumables with a calendar: price the replacements, not just the install. A neglected filter is worse than no filter.