There is another way of looking at this – if you effectively buy common property (the wall your balcony is attached to and the air space the balcony occupies) you have to pay for the building work AND compensate the owners corporation for the use of common property on a formula based on how much value the balcony will add to your property.
There might be a nifty quid pro quo going on by keeping this as common property … so be careful what you wish for. And I doubt if you'd be splitting the costs evenly – it will be based on your Unit Entitlements, won't it? Or are some units not getting balconies?