Do you need so many speaker pairs? If it's for critical listening then more than one pair in a room will ruin it and if you're just doing background muzak-y things then wouldn't a radio-style unit suffice?
Can you link the chromecast audios to stream to multiple rooms at once?
Good point. That is the number of speaker points in each room, so don't actually have to put that many speakers in. That would reduce the investment significantly, if I just went with two speakers per room I could go down to 16 speakers...
Ouch. that starts adding up...
tempted to just throw out £30 rubbish ones to start with and upgrade room by room.