The aircraft - belated replacements for the Vietnam-war era Caribous retired in 2009 - will cost the taxpayer $30 million each, for a total of $300 million.

Defence, which has refused to provide a detailed cost breakdown, says the remaining $1.1 billion will pay for ''modifications to the aircraft needed for specific ADF roles, initial logistics support (including spare parts, training, materiel handling equipment, technical data and management fees), testing and certification and facilities''.

''These costs would apply to any aircraft platform chosen, including the C-295,'' a Defence spokesman said.

The Canberra Times has been told the electronic warfare, missile warning, countermeasures, radar, communications, identification friend or foe, trackers and mission planning systems will cost between $5 million and $6 million per aircraft.

The C-295, an Airbus Military product, was beaten out for the contract by the Spartan.

Airbus Military, which argues its plane could have delivered a near equivalent capability for less cost, recently rejected claims by Defence Minister Stephen Smith that there had been a fair and open competition.

Sources have told The Canberra Times the decision to choose the Spartan may have been made months before the formal announcement on May 10, just days after major Defence spending cuts were unveiled in the budget.

Defence won't say when the agreement to buy the Spartans was reached or when the contract was signed. ''The Defence Materiel Organisation has signed a contract with the United States government (through a letter of offer and acceptance) on behalf of the Commonwealth,'' the spokesman said.

L-3 Communications, the US intermediary handling the sale through the US government, also refused to say when the deal was done.

''These are questions best put to the Australian and US governments,'' a spokesman said.

America's Defence Security Co-operation Agency (DSCA) notified the US Congress of the possible foreign military sale of the Spartans and associated equipment, training and logistical support to Australia on December 16 last year, five months before the announcement at the RAAF Air Power Conference.

The DSCA notification says the total price of the US technology being sold to the ADF - including the planes, all electronic systems, spare parts, ferry costs, training, manuals, a flight simulator and the like - is $950 million.

This is $450 million short of the $1.4 billion quoted by the Defence Minister.

Mr Smith's $1.4 billion does not include through-life support costs. ''Defence will seek a separate agreement with Alenia [the manufacturer] in order to ensure RAAF can operate, maintain and modify the aircraft through its planned life,'' he told the conference.

The Defence spokesman said the $450 million would be spent on ''facilities, Australian logistics systems (to load and unload the planes) and further test and evaluation for the Australian configuration''.

Industry sources are at a loss to work out why this would cost as much as a new base hospital and radiotherapy centre for a major provincial city.

Asked to comment, Defence said: ''[We] are not able to provide you with commercial-in-confidence costs [or] confirm your sources or their estimates''.

The mystery millions won't be going on new hangar space. Defence confirmed the C-27Js would use hangars at the Richmond RAAF base to be vacated by the 12 C-130H Hercules being retired this financial year.

''Costs are expected to be small and are yet to be finalised,'' the spokesman said.