Im pretty sure the organizations could come up with proper figures if they wanted to or saw need to do so, given the fact that they do keep records.
However, they have to do polls to find out how many certified divers there is that quit every year as well to find the number of active ones.
As far as money goes, I think it might be rather insignificant. Countries with lower income also have lower prices. They might not go to hawaii to dive, because they cant afford it, but they can always dive locally. Location and the tradition of diving I would think has a greater impact on the number of active divers in an area.
For statistics youd juts need to know how many divers are currently certified with each organization and that should be possible and not very hard to find out, if the organizations so decided.
If you want active certified divers (guesstimate) find out how many percent quit each year, take it off the number of certified divers each year.
And then comes the part where organizations realy would shoot as blindly as anyone else.. How many UN-certified active divers are there? My guess would be "a lot"..