Be sure that some people in fact paid you the $30 only because they tried it first and said "this is worth it" (it is how it sometimes actually works).
Meh, that's an argument often heard "I'll download this game to try it and if I like it I'll buy it legit", but how often does it really happen? :). Maybe some did, but it's impossible to quantify.
Someone that downloads something and never pays for it is no different than them never downloading it but also never paying for it. They're non-customers.
There's also no difference between the game downloader and them playing a copy at their friend's house and deciding not to buy it. There's also no difference if they were to buy the game second hand.
I am not sure about the rhetoric: why should you quantify it?
If it is to calculate missed revenue: are you considering that the other side of what I have written - a fact - is that a number of people who downloaded an available copy of your book would not have bought it anyway?
Those relevant to the missed revenue are those who would have paid but did not: the number in that group is something that you can hardly quantify, and on the other hand you have gained buyers that would not have bought your book if they had not tried it.
Be sure that some people in fact paid you the $30 only because they tried it first and said "this is worth it" (it is how it sometimes actually works).