A 500W kit costs around 500€. With good orientation you break even in 3.5 years (2kWh per day at 20c the kWh). Very few investments would give you such 25-30% yearly ROI.
They are getting even cheaper. I have four modules with a total capacity of about 1800W (the inverter has a limit of 800W to comply with the regulations). Cost was about 370€ including delivery.
Sure, but how much € can you actually save after 3.5 years per year?