I hope you’re not implying that Apple uses the same logic boards in both products. I’m sure there is processor similarity (and BTW would be similar to the Air, NOT the MacBook which uses “Y” ULV CPUs) but I would have to believe Apple doesn’t carry such inventory that they have to make other products to ship them.
I just typed macbook casually for the whole notebook category since I thought it is well known. That's really out of my expectation. Ok, Macbook Air and Macbook Pro (Retina).
By the way, the boards cost nearly nothing. Only the stuffs on the board cost something.