Only less than half if you ignore the land costs...Firstly, the cost to build is usually significantly less than the market value, sometimes less than half.
Secondly, right to buy has caps and those eligible will have had to qualify with points that are geared to the most needy
On the other hand if a council was able to build on land that it (i.e. the public) owns, then that would make sense in building council houses.
There are many reasons why they effectively can't do that, but mainly that they have to provide mandatory services which are not fully funded so are forced to sell off assets. In this case the land.
Plus developers are ripping off councils en masse, and in some cases maybe in collusion with...
As an example, I heard of a small development which had to include a few so called 'affordable' units.
After agreeing the developer came back and said they couldn't do it so it had to go before an independent tribunal.
The amount they needed to pay the council was about £800k.
A while later they came back and said all we can afford is £25k and this was accepted by the council.
So no 'affordable' units at all and a big win for the developers!