Yet another data point: ten years ago it was £5,500 for a 3.9kW system. The largest possible on the "simple" permitting scheme. I did however have to spend another £1000 a few years later on retrofitting bird-proofing. You don't want to build a pigeon farm by mistake.

At that date I got into the feed-in-tariff scheme, and it has long since paid for itself just in cheques mailed to me by the utility.