If your starting point is a direct port of Typhoon avionics, this means that you are not developing an airframe and the systems at the same time. I'm assuming that this would significantly reduce the initial costs, and the systems can be upgraded as an ongoing thing once the airframe is in the air and in production.
Would be interesting to see a rough breakdown of costs for the F-35 (A only) development.
AND
Yes, that's my take on it. Typhoon avionics will be upgraded over the years, & the latest iteration (perhaps incorporating input from Tempest partners) will be ported to Tempest when the airframe & engine are ready, to be replaced by new developments later.
Isn't that pretty much what was done with the F-18C/D & E/F?
Honestly if the goal is a 6th gen fighter aircraft, I would be worried about the effectiveness of such a strategy.
The avionics used in late production F/A-18 Hornets had a high degree of commonality with Block I (early production) F/A-18E/F Super Hornets, but not with Block II Super Hornets. As an example, the APG-73 pulse-doppler radar fitted to the C/D model Hornets was also fitted to Block I Super Hornets, while Block II/III Super Hornets and the similar Growler are fitted with the APG-77 AESA radar. AFAIK the APG-79 was not backfitted to Block I Super Hornets manufactured with the APG-73 because the aircraft nose could not accommodate the newer radar. I have heard a rumour that there was to be an upgrade to Block I Super Hornets to fit them with the APG-79, but so far I have not been able to confirm that, and I have come across contracts from this year where the USN contracting for upgrades to the APG-73 still in use on some Hornets and Super Hornets. The USMC also put out a RFI about replacing the radars on legacy Hornets still in service with a new AESA radar, also from earlier this year. Given that one of the possible contenders was a Raytheon entry based off the APG-79, that does indeed sound like the space for an APG-73 is inadequate for an APG-79.
The reason I mention all this is that with the commonality of avionics between late production Hornets and early production Super Hornets, one is dealing with avionics commonality between different aircraft of the same generation (4th), and it is not really until one gets into the Block II Super Hornets that it would really be considered a 4.5 gen fighter IMO. The Typhoon's are also 4th or 4.5 gen fighter aircraft and expecting to be able to 'jump' an entire generation and then some IMO will not work.
Once specific systems and configurations are developed, they might be able to get installed and tested aboard a Typhoon, but that is about it I suspect. As a side note, IIRC during some of the development and testing for the F-35 avionics, a special Boeing 737 was modified to serve as a testbed for some of the avionics configurations
I would also be interested to see what the definition and expected/planned capability set for a 6th gen fighter will be.