The New York Times published its Q1 earnings report last week, and reported an 18% year-over-year growth in digital subscription revenue.While print circulation continues to tumble, NYT had 799,000 digital-only paying subscribers by the end of Q1, earning the Grey Lady $40 million in revenue. In addition, NYT added more digital subscribers in Q1 of this year than any quarter of 2013, a health show of growth for a subscription-based business.While most news outlets focus on the year-to-year growth, however, we would caution analysts from getting a little too giddy over subscriber acquisition. As any good subscription marketer knows, the real value of a subscriber is realized over the lifetime of a subscription. This fact is augmented in the digital space, where the cost of fulfilling subscriptions is often cheaper than in print.