What is the religious ruling on the exorbitant costs of building mosques?

The construction and maintenance of mosques, as far as is necessary, commensurate with their status, and facilitates worship, is a religious requirement, provided it does not reach the point of extravagance and wastefulness without real benefit, such as decorating them for ostentation. This is blameworthy and forbidden by Islamic law. The Prophet, peace and blessings be upon him, built his mosque with tree trunks and palm fronds as evidence of abandoning excessive ornamentation and extravagance, and he forbade their excessive decoration and grand construction.