There isn;t actually a higher proportion of Sceintology celebrities to non-celebrities then you would find in any other religion. As a Scientology staf member for many years I can tell you that most Scientologists are not particulary rich, nor are they all celebrities!
The Church does operate several Celebrity Centres, who simply offer the services of Scientology to celebrities in an environment free from interruptions and distractions. The definition of celebrity used here is very broad - it includes artists, performers, muscians, business leaders, their familes and entourage.
You certainly do not need to be rich to study Scientology - you can find them at http://course.volunteerministers.org/login/
What makes a Scientologist is not how much he has speant or anything like that, but rather how he uses that information he has learnt to improve himself and the lives of those around him.