You would need to modify the airframe & intakes for new engines-same applies for adapting a radar.All those modifications would make the new F-15 costs go up by a factor of around 1.5 times on average.An export customer maybe willing to gobble it up,but what about the USAF??
Not necessarily. The F-14 was re-engined without major airframe or intake changes. It really depends on the size of the new engine and whether it is developed specifically for the use or is in existence. The radar has already been changed on the F-15 more than once. Again it comes down to space available versus required. As the F-22 radar is fixed array and the nose section is not larger than the F-15, installation may not require any major airframe changes at all.
The F-15K is very different from the F-15C, but is not 1.5 times the cost. Adjusted for inflation, it is about the same.
I could see the addition of vectored thrust creating a significant R&D charge, but then again maybe not, as it has already been done with the F-15 Active Program.
Again my point would not be to replace the F-22, but to flesh out the force with a Cat 4+ Strike-Attack aircraft that would be equal or better than the SU-30 and Eurofighter.