Let's consider a family consuming six litres of bottled water a day.  

Six litres of bottled water per day for a year = 2200 litres weighing 2.2 tons.

At 0.30 euro per litre it costs 660 euros per year.

Suppose they install a FILTRATION KIT costing 152 euros, the initial cost (investment) is amortized in about four months.

After that, they have only to pay for the replacement of the filter cartridge.
It costs 50 euros and is good for a nominal 5000 litres.
Nominally then filtration costs 1 eurocent per litre.

We must now add the price of water supplied by their water company -
it's typically 10 euros per cubic meter = 1 eurocent per litre.

Then the nominal cost of their filtered water would be
1 eurocent for filtering  + 1 eurocent for the water = 2 eurocents per litre.

In practice the capacity of the filter depends on the quality of the water supplied by the water company. We also recommend that the filter be changed every year. So the cost of filtration to the typical family using 2200 litres per year might be about 2.3 eurocents per litre.

Under these assumptions, their filtered water would cost  2.0 to 3.3 eurocents per litre.

This is to be compared with supermarket bottled water at 30 eurocents per litre.

For one year, filtered water would cost  2200 litres  x  (2.0 to 3.3) cents per litre  =  44  to  73  euros per year

To be compared with the cost of bottled water: 660  euros per year.

Cost	in Euros/litre
Bottled water	0,300 Euros/litre*
Filtration kit	0,025 Euros/litre*

* Hypothesis : one litre of bottled water worth 0,3 Euros per litre, one m3 of tap water worth 10 Euros, filtration kit worth 152 Euros amortized on 30.000 litres, one filtration cartridge worth 50 Euros changed every 5000 litres (duration of cartridges may vary depending on the quality of water).

Filtration home page - Some facts about water filtration - The chore of buying and carrying bottled water - Filtration brochure (PDF printable format) - Order form for a filtration kit - Order form (PDF printable format)