It’s around $450 million according to financial site, 24/7 Wall Street. They based the calculation on the basis that for every one legitimate paid sale there are three illegal downloads. That equates to $4.5 billion. Unlike the music industry’s vastly inflated claims, the calculation sensibly assumes that at best, 10% would have paid for those apps if they could only get them legally. That’s how they arrived at the $450 million.
I’m not too worried about Apple as they are doing very nicely from the iphone and app store. There will always be piracy, but as always it will be the small developers who suffer the worst.