At the current $15 a month, the WSJ Kindle subscriptions represent about $5.4 million annually. But it’s not that easy: the subscription price and the revenue split with Amazon (NSDQ: AMZN) shifted earlier this year; the price increased 50 percent from $10 while Amazon’s share of the take dropped. (The issue with customer relationships, a pet peeve of Rupert Murdoch, didn’t change: Amazon still controls the finances and the names.) I’m told that roughly half of the subscribers are still at the previous rate. That would put the current overall revenue at about $4.5 million, with Dow Jones (NYSE: NWS) getting less than half that. Incremental, yes, and it will continue to be. But it also should continue to grow.

The hardest bit to decipher at this point is how many people are giving up print or WSJ.com subscriptions for e-readers and, now, the new mobile subscription.