(Edited in response to John’s comments.)
The number we’re looking at is total paid digital subscribers to the NYT — not including print subscribers or people who get their digital subscription for free through some promotion. We’ll look at whatever public statement the NYT makes about that number on or near the second anniversary of the paywall, which will be March 28, 2013.
Let the number of paid digital subscribers to the NYT on that date be X. Then I owe John $(X-300,000)/1,000. If that number is negative, then the sum is owed by John to me.
If X=0, ie if the paywall is dismantled before its second birthday, then by the formula John will owe me $300: that’s his maximum downside. My downside, by contrast, is unlimited: if the NYT gets 1 million paying digital subscribers, I’ll owe him $700.