The draft NBNCo Corporate Plan (2013) contains this statement:
"As at 30 April 2013, 26% of NBN Co’s FTTP End-Users were on the highest available wholesale speed tier (100/40 Mbps), whilst 47% were on the entry-level wholesale speed tier (12/1 Mbps). These compare with 18% and 49% respectively forecast for FY2013 in the 2012-15 Corporate Plan."
Many Australians have voted with their wallets and decided that 12Mbps is adequate. Optus have identified this and I would suggest their financial modelling suggests that they can deliver a 12Mbps wireless service for less than $25/month of NBNCo wholesale charges.
When NBNCo's take-up rates fall, it's costs barely change because the fibre has already been laid, but revenue falls which will drive up wholesale charges.