Sky Sports has come closest to the method I suspect MoD use...
In reality, these aircraft tend to be acquired as part of a programme, and involve elements of risk and development cost sharing.
So, unlike when you buy a car which has a sticker price of say, £15000, with the manufacturer bearing all costs associated with getting it to the showroom and being responsible should it sell 1 or 100000 examples, aircraft procurement is typically much more complicated.
To illustrate, say the RAF identify a need for a new aircraft type. After determining that dear old BAE are the chosen contractor, a contract is entered to develop and build the Albatross, at a cost of £100m (if only!) for 100 aircraft. So far so simple, but MoD point out that there will be overseas orders for such a wonderful aircraft, and since £50m if the £100m was actually design and development cost, they should benefit from overseas sales, so BAE agree to give MoD £250K per overseas sale. 100 overseas sales are forecast.
What is the cost of the aircraft at this point, £1m or £750k?
Once development work begins, cost overruns are incurred achieving the specified vertical takeoff and landing performance (sound familiar?). AS this was so revolutionary,sic, MoD have to pick up 75% of the 50m overrun. Different cost per aircraft again.
Then MoD ask for a new avionics fit and 100kt higher speed. 2 year programme overrun, with all costs to MoD. Aircraft now too sophisticated to be sold to all but our closest allies, overseas order forecast now cut to 50 aircraft. Different cost per aircraft.
Two aircraft destroyed in testing, programme now cut to 148 aircraft. MoD decide to lease the fleet from BAE finance inc of Grand Cayman at an annual cost of US$ 15m a year for 15 years after which there will be a charge per remaining aircraft of US$1m. 8 aircraft are forecast to be written off during this period.
Australia cancels its interest in the Albatross and buys F 16, overseas order potential cut to 25. And on and on.......
All entirely whimsical, but perhaps allows you to appreciate why Sky Sports wasnt far off the mark! Cost per aircraft is a somewhat arbitrary number at the best of times.