According to an article in the Guardian:

Illegal downloading of music cost the UK industry nearly £1bn this year, the BPI claimed today, as it produced research showing online piracy is still growing.

…

It based that calculation on an assumption that every track would have sold for 82p, the average price of a digital single, although it conceded not everyone who downloaded tracks illegally would have paid for them if they had been unable to obtain them illegitimately.

Also, last year thousands of people rode on the same bus as me. This cost me thousands of euros. I base that calculation on an assumption that they would each have paid me €1 to keep them company, though I concede that maybe not all of them would have paid if they could just have gone along anyway. As indeed they did.