Everything is relative. Using a PV system can be more expensive than buying power from the local utility, through the electrical outlet in your wall.
Every application is unique, and generalizations on cost are difficult to make.
How much PV you need depends on your power loads and their duty cycles. If you wanted to completely replace your current electrical purchases from the utility with a PV system, you could look at your kWh usage on your electric bills for a year, calculate a daily average, and divide that by the number of average daily sun hours for your location. However, if you design an energy efficient home, you could cut the annual electricity usage dramatically, reducing the size of the system. In the real world, the majority of home systems range from 1 kW to 2 kW. Where you live, if you are on the grid or off, and how you live, will dictate the size of your system, and its ultimate cost and value. PV modules have been tested in controlled settings and in the field, with results showing module lifetimes in excess of 20 years.