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. PV modules have been tested in controlled settings and in the field, with results showing module lifetimes in excess of 20 years. The majority of power modules in use since 1955 are made of single- or multicrystalline silicon, though several manufacturers are producing large quantities of amorphous silicon power modules. There are projections of large markets for utility grid support and for building-integrated PV systems in developed countries.