Hi,
I've been doing a little research in the Timewyrm and Hunky Monky files
as to the most common causes quoted by players for dropping. Yes. money
is fairly important, mentioned by 24% of people who gave any reason for
dropping at all. However time of one sort or another is by far the most
common reason at 62%. (included in that figure is a fair numberof "my
wife/girlfriend says I'm spending far too much time on PBM" stories) 
I would assume this sort of figure might be representative of quite a
lot of PBM. Let's face it, a large proportion of PBMers used to game a
lot more than they do now and no longer have the time (work, marriages
etc) to do face-t-face roleplaying, boardgaming, etc, so play PBM. Time
is what got a lot of us into the hobby and thus it remains the biggest
restriction.
Those of us who are GM's on the other hand have infinite time (a
necessary quality to be a GM) and no money (a result of being a GM).
Colin.