if you want new parts then your looking at £1200ish to put together your own kit with porsche calipers being the most expensive part (theres a porsche spares website that sell the exact calipers, new for £400 each), i priced up the job last month but never went through with it, you can buy kits from the likes of awesome for £800 but they dont come with pads.
As for the full job done for £500, you might have done yours for £500 mate but that doesnt nessesarily mean eveyone can.