The F-4K (FG1) and F-4M (FGR2) did indeed end up costing 2-3 x the cost of a F-4J.
However:
- The F-4K could not have operated from HMS Ark Royal and HMS Eagle without the more powerful Spey engine - 2 x 20,500 lbf vs 2 x 17,900 lbf of the J79 (the HMS Eagle refit to carry the F-4K was subsequently cancelled).
- The UK government of the day insisted on over 50% of the manufacturing work going to UK industry which had just suffered the cancellation of P1154, HS 681 and TSR2.
- The Spey turbofan - as opposed to the J79 turbojet - was also the better choice for the low level nuclear strike / attack / recce role the F-4M was originally employed in.
- The government had fixed the funds available for F-4K/M procurement - originally 400 were planned - but when the costs rose numbers were cut to 170 - which in turn drove the unit cost up even further.
- Had the original 400 been procured, the unit cost would have been much lower and the RAF probably wouldn't have had to make the retrograde step of turning the Pussycat advanced trainer (by flip flopping the relative production numbers for planned 2-seat / single-seat variants) into a less capable pseudo strike / attack / recce aircraft to "replace" the F-4M to free up the numbers required to replace the Lightning for UK AD !