It wasn't donated money entirely - at least not in the sense of offerings from regular devotees. They were donations/remittances by the monarchy. The history is that the southern half of Kerala was a kingdom named 'Travancore'. During the establishment of the kingdom, the first king declared himself and his successors to be 'Padmanabhadasa' or 'servants of lord Padmanabha'. Lord Padmanabha is the deity worshiped at this particular temple. The temple was considered as the seat of Travancore's power and therefore a big part of the kingdom's tax revenue ended up there.