With the exception of Hasiddic synagogues and Chabad, to the best of my knowledge, all synagogues have membership fees, usually proportional to the income of the family. In the case of Hassidic synagogues, it's just assumed that the family will be giving generously to charities, as well as the synagogue. In the case of Chabad, they'll happily take donations, but everyone is welcome, no matter what (within reason, of course).
Mark
Wow! Thanks for the info. I did NOT know that.