david cameron announces extension to right-to-buy housing policy .
pm promises to extend right to buy to all housing association tenants .
discounts of up to 70 % to allow 1.3 million families to buy their home .
national housing federation said the subsidy will cost taxpayers £ 5.8 bn .
claimed it was effectively worth £ 100,000 for each family who benefited .
but the tories said it would be funded by making councils sell off homes .