Lior,
So MO shuls charge so much more because they pay their rabbi so much more than chareidi shuls and they have larger shul staffs?
Not sure you can pin it to MO vs. non-MO shuls, but your reasoning is basically correct. Different shuls have different needs or philosophies about what services a shul will provide to its community. A shul with the sole purpose of providing a small space for men to daven will have the lowest overhead costs. That setup is feasible in areas where a frum infrastructure is already well established. Other shuls require the full time participation of one or more rabbis and other staff to provide additional services that the community wants or needs, and they may have a larger or more expensive building.