To answer your question I refer you to the many documentaries on poundland! You might think they make very little, but it might be a high margin. If it costs £1 per item to sell, the factory probably have a cost of 10p etc.
Its not impossible that B&M are selling at a loss or break even at £5. The model of shops like that is to make a target of xxx from a range and once that's met they can discount to whatever - some people have bought sets for £1. B&M is based on buy and sell quickly and keep the shleves full.
It would be fascinationg to know the actual real £££ though wouldn't it?