There're 8 1 bedroom flats in Clapham on the market on rightmove that have a starting price of at least 650k. At least one is listed twice with two different agents. The total amount of 1 bedroom flats listed in Clapham on rightmove is 272. Correct me if I'm wrong, but this is about 2.9%. This is far from the assumption that "a one bedroom flat in Clapham would cost 650k".
Again, we are talking about construction costs and not sale prices. The difference in building a 1st floor flat compared to a penthouse flat of roughly same size will be much lower than the difference in sale prices.
While I understand that the real estate prices in London are extortionate, this is not due to construction costs, but due to high demand. For big developments, in London, the targeted sales revenue at 100% sold is about twice the building costs (excluding solicitors, estate agents etc).