I can think of a few reasons for the difference. Unlike Christianity, which can be spread by way of "the good news" and a conversion experience, Scientology requires individual training in the methods. Money does two things in the set up. The first thing it does is separate the pretenders from the serious students. The second thing it does is support the organization.
Rather than rely on a broad pool of people who generally support the church, it focuses on a smaller elite. You might think of it as akin to priests or pastors -- those who are more highly trained. Certainly, other religions do charge to become elites in their practices.
I have recently come across a couple who have applied to get missionary training in the Protestant/Baptist tradition. They have to raise enough money to pay for the schooling and then, afterwards, they have to raise money to fund whatever mission they are assigned to go on. There is a great deal of money involved, at least thousands of dollars. They could, I suppose, come up with this money themselves instead of asking for donations. The point is that money is necessary except for those who are not really committed.
This is not to say that everything Scientology does costs people money. I'd encourage you to investigate free programs and charitable works. I think they receive little press.
