A pool pump for a standard size pool consumes between 750 an 1000 watts every hour it's running so if you allow 20cents per hour you won't be far off. Normally six or seven hours of filtering is sufficient so again say around €1.50 a day or close to €10 a week. If you have a chlorinator (salt water pool that makes its own chlorine) you will need to run a few hours longer each day in Summer. Probably around 10hrs. Swimming in a salt pool is nicer on balance.
If you have a pool cover you will need less filtering in Winter and can cut down the pump time to a couple of hours twice in any 24hr period.
Very easy to add 3 or 4 Photovoltaic panels (latest varieties generate 325watts each) to generate a reliable quantity of power for the pool pump and run it free when the sun is out.
A replacement pool pump costs €250 to €300 euros and they last five to eight years. They can be rebuilt once (new bearings etc) but often its not worth it and only adds a couple more years to the life of a pump.
The sand in a sand filter should be changed every five years or so as it gets hard and loses its filtering ability.
Watch out for tiled pools needing 're grouting', the jointing material doesn't last for ever and doing the job properly costs a few thousand euro.
A bucket of pool chlorine lasts a season and costs around €30 but if you fill with water from a borehole (normally quite alkaline) then you will need to balance the pH and a bucket of acid that will last the season costs about the same.
Buy a test kit and make sure first that the pH stays correct, with that done then the chlorine level is easier to keep stable.
Buy a pool net and train all your swimmers to clear bits from the water to help keep the filter more clear of debris.
If you choose to appoint a local pool service then they will charge €80 to €100 euros a month including chemicals. These guys however tend to prefer to leave the pump running 12hrs a day or more as it makes less work for them - so watch out for that one.
Not a low cost amenity but its one of the nicest things you can have !