Tennis as a job

Sorry if this has been asked before, but I was wondering whether any of you know roughly how many tennis players can be considered "pros", meaning that they earn enough money from playing tennis for a living? My guess is the top 200 players maybe? Less? Or more?

Thanks a lot for the info, it is just a topic that I have been wondering about for a long time...

There are two threads that deal with this and I will bump them for you.

http://www.menstennisforums.com/showthread.php?p=6535754&highlight=Ladder#post6535754

Great, thanks a lot! Very interesting read "(the ladder to success")... so the treshold seems to be between 100 - 200...