My shul has a base amount per family, plus a small amount per child. My membership this year came out to about $1,200.
The membership doesn’t just cover seats. They have youth groups every Shabbos and Yom Tov during Shacharis, where the youth leaders teach the children parts of Shabbos davening and discuss the parshah with them, so the parents can daven without being disturbed. There are groups for the kids on Shabbos afternoon. There is a kiddush and Seudah Shlishi in shul every week. The membership dues cover the bills the shul has.