Planes (F-35s) are about 10-15x the cost of a submarine (Virginia Class). So you're talking about ~100 planes.I'm not sure what good 8 subs will do. 100's of Planes and ships would be better defence and much cheaper.
Submarines have more range, harder to detect, can basically operate anywhere in the world without support, more firepower (10x more missiles).
HMAS Adelaide cost more than half of the cost of a Virginia Class Submarine.