I can't tell you the cost, because our water meter was installed by the builders of our house (in 1994). I do not think that there is much inconvenience, as the meter is installed outside the house, at the end of the driveway, or some such place that is convenient for the water board. As regards the "running costs", yes, it is relatively cheaper for a two-person (not necessarily elderly people!) household to pay for water via a meter. The usual rule of thumb goes something like this:
the fewer people in the house, the cheaper the water is. My wife and I did not have a meter in our previous house, but, since moving to this house (with a meter) we are paying less for our water.