Cavity sliding doors were only $330 or so for us though going from memory, we put them in for the ensuite, study, lounge/hallway and lounge/rumpus rooms.
We raised our ceiling height as well, so all our doors are more expensive as they all need to be taller otherwise it would look funny - not something I thought of when we wanted the higher ceiling.
We went with the Dream Life Indoor Promotion Pack so have 2,590mm high ceilings rather than the standard 2,440mm, I would have thought the cavity doors would be standard height anyway?