Costs for under sink filter systems range from 100 euros DIY to around 600 Euros nowadays for a full 5 stage RO with booster pump installed, so they beat bottled water hands down.
Filters can only improve water quality if correctly chosen and with reasonable service intervals for filters the costs can be kept as low as 10 Euros a year for simple single cartridge systems for sediment etc, to around 100 Euros for RO servicing every year.
Single housing sediment filters can act as a grit and sediment barrier for things like helping to avoid attic ballcocks from sticking, and a mild improvement for the drinking end of things.
Single activated carbon filters are a good all rounder for chlorine reduction and odour, colour, taste reduction. Carbon block cartridges have an added solid microporous structure to stop particles from 10 micron, to 5 micron, also 1 micron and 0.5 micron available.
Better still instead of 0.5 micron carbon where small stuff like bacteria and turbidity are concerned is a two stage arrangement with 10 micron or 5 micron carbon block followed by a 0.5 micron nominal Doulton ceramic "Sterasyl" cartridge. Low cost to buy and possibly 150 Euros DIY for the kit or up to 300 Euros supplied and installed with back up service. You get high flow through the two cartridges than one finer cartridge "all rounder", little in the difference for service costs and more even and longer lasting filtration.
Always get the universal 10" cartridges with the above kits and you will be guaranteed low cost and easy to get replacement filters. The above kits work on hard water so you still get the scale in kettles and often overly high lime content / calcium, but full minerals are kept in the water if that rocks your boat.
Better still for excellent filtration the RO type system is unbeatable, from 3 to 5 stage, pumped or unpumped, from 300 Euros, 3 stage DIY, or 450 plus installed, or the full 5 stage pumped from 400 Diy, to 500 Euros or 600 Euros fitted depending on models.
RO systems fitted directly to hard water are not the best way to go about things but they will work happily enough and servicing will cost maybe 100 Euros for a 3 pack pre-filter service or 50 Euros DIY each year and then every 3 years or so an extra RO membrane change for another 50 Euro DIY or 75 to 100 Euro fitted or less for all 5 stages pro rata. The water will have good mineral content after filtration despite some plumber's thoughts on the matter, as much as Volvic bottled mineral water when working on hard waters like in Galway County.
RO systems fed off a pre-treated supply especially a water softener give excellent extended life to the RO system. They take out all sodium so no worry there, (also sodium in soft water is as little as bottled water anyway and no chloride), with this arrangement the RO will usually last 5 years or more between membrane changes and the pump is extra protected from risk of scale erosion on the diaphragm, so no early surprises on failed pumps which can cost 100+ DIY to 200+ supply and fit. Also pre-filters are far better protected from dirt build up that softeners regularly backwash out of the incoming supply, so pre-filters last up to 2 years in many cases.
So there you have it, single, double, triple RO and five stage RO systems and all the costs and servicing.
All systems beat bottled water hands down, especially if you are paying the average 500 a year on bottled water, even down to 200 euros a year (over a longer period allowing for the highest level system cost and service costs.)
If you like your tap water or bottled water carry on drinking, but have a thought about the mad costs of bottled water, carrying it, disposing waste to land fill, and the fact we have been trying to drink clean water one way or another for the last 10,000 years or more without buying bottled water, only really since the 1970's and 1980's after the advertising got it's claws on the idea and we all got hooked on something a little bit daft. See Penn and Teller's take on it on Youtube, funny and informative stuff.