Maybe try looking at the world population growth vs Jehvah's Witnesses membership growth.
Jehovah's Witness annual growth is currently 2.4% (see: http://en.wikipedia.org/wiki/Jehovah%27s_Witnesses)
Worldwide population growth per annum is 1.4% (see: http://www.wolframalpha.com/input/?i=world+population+growth). Maybe JW born-ins are more than 1.4%? I know they were in my congregation. But to be fair we will take the 1.4% official figure.
So Jehovah's Witnesses are growing at 1% above general population growth.
This 1% growth rate is accounted for by the preaching efforts, which total around 1.2 billion hours per annum. That's 4,054 hours per convert. Image if that was paid work, it'd be somewhere around $40,000 to $80,000 per convert.