Here's an article that ars just posted yesterday in fact about that.

Tldr some group states that piracy affects company's bottom lines by up to 20%. They also state that since game sales figures are almost never published, they had to derive a replacement by using reviews and active player count. To me that just kinda invalidates the whole thing because who the hell knows what the actual relationship is between those variables, and especially when it's making such a gargantuan claim that pirates are taking such large chunks of cash from developers pockets. If companies want to show they really are being hampered by piracy to such a degree, they should post their actual books and stop hiding key information.