What made the F-22 and F-35 expensive is not their bodies but their avionics. From the shaping perspective, the transition from the F-117 to the F-22/35 was revolutionary. But what made the F-22/35 expensive was what we wanted each platform to do. The F-22 do not need thrust vectoring to be 'stealthy', but we wanted TV just in case we got into a close-in fight and it is those 'just in case' scenarios that ended up adding to the final cost.
Same thing with the '4th-gen' platforms. We want to extend their service lives as much as possible so we gave them upgraded avionics that ended up with something somewhat silly like '4.5 gen'. Take AESA radar, for example. The first time I saw an AESA radar in action, albeit from the ground, I knew the US would skip PESA and I was correct. The capabilities leap from PESA to AESA was exponential and so was the cost. Data fusion have nothing to do with 'stealth' but it was technically easier to give the later platforms data fusion than it is to make incremental bloc changes to the current '4th gen' platforms. The end results are the shocking sticker prices for the F-22 and F-35, but it would have doubled to gut the F-15 and give it data fusion capabilities. Better off to pay the high prices now and mature the technologies on the latest platforms than to spend yrs experimenting with the current ones.
Imagine this...I have 100 F-15s. Now I have to figure out how many to remove from combat ready status to spend X months in upgrade. So why not develop a new platform with all those fancy capabilities to give the fleet a technological leap? We did have thrust vectoring (TV) on an experimental F-15: STOL/MTD. Can we have TV, AESA, and data fusion on an F-15? Absolutely. But how much would it cost -- time and money -- to retrofit the entire fleet? Not only that, how much of the fleet is possible due to time and fatigue? If an airframe is past the mid-way point, is it financially sound to modify that jet?
In the end, it is better to take all the things you want and put them into a 'wish list' for the next generation.