PMAX, I am feeling giving today so I will hold your hand and walk you throw some basic facts. Take that hand I am holding and count it, yes that is one hand, that is how many camshafts the 912 has. Now hold both your hands out and count them, that's right you should have two hands, that is how many camshafts the 911 engine has.
So if you had to replace this on your engine guess what will cost more? that's right two camshafts cost more than one.
Now let's go to pistons and Cylinders a basic components of an average rebuild and we can get hard #'s right here on Pelican parts
for the 912E P&C's cost $359.25
1976 Porsche 912 E Coupe - Pistons and Cylinders - Page 1
the 1988 911 P&C's cost $3289.50
1976 Porsche 912 E Coupe - Pistons and Cylinders - Page 1
We can go on, but I think you should be able to get it, if this is not clear enough of and example then there is nothing more I can really do.