It's a number that may be impossible to account for. Heres a study concerning illegal music file sharers, it turns out that they spent more money on music than people who don't share music illegally.

http://www.independent.co.uk/news/uk/crime/illegal-downloade...

Just like radio (play for free) increases album sales for record labels.

This suggests that "lost sales" may mean a product that is not worth buying to some.