I kind of treat the 'value' in the 2800 vs 3.0 in the way I would treat 2002's.....
Condition, condition, condition....... (and bumper configuration...
)
I had been looking (off and on) for ~10 years for an E9. I felt I understood the differences well enough and wanted (in a perfect world) a '72 or '73 3.0CS
I favored the larger engine and brake set up. That and I figured a couple of years less age couldn't hurt either....
So - I'd say....
Treat all the E9's the same. My only real issue was the bumpers - The E9 had larger bumpers at the end (as did the 2002) so I kept my search to the earlier cars.
But aside from that - Next on the list was transmission, A/C and sunroof.
I actually wanted a car w/o sunroof. No drain problems to worry about.
I also wanted a car w/o air conditioning. Easier to work on.
Transmission - I wanted a 4 speed but would consider automatics as I could always convert to 5 speed later - But the car would have to be VERY appealing otherwise.
Then I ran across Tony's (Velocewest's) car in Portland.
His was a '70 2800CS in tremendous shape.
While I had previously driven the 3.0CS (carb) I had not taken a 2.8 out.
So - My original criteria as you can see was tossed aside when I saw Tony's car.... His car has air.
But the rest of the package was well worth it.
So - I think the 'value' in the car lies in its condition... with all the other factors playing second fiddle to that (even CS vs CSi discussion).
As to what that 'value' is...??? That is up to you and the seller.
I have seen some cars cross at very high prices and others for less than I would expect.
Good luck -