You are right the Supplement Greyhawk is not a part of the original contents of the
OCE White Box Set. Or any of the original box sets in fact.
I agree with the others as to the VERY subjective value of the
OCE set. Unless the box and books are in mint or at least near mint $60 is all that I would put up for one, but they often seem to get $75, or even $100, for books near-mint and the box good to very good.
A Mint box and Mint books however, have gotten much higher prices on E-Bay in the past couple of years. I watched a shrink wrapped copy of the
OCE go for over $225 about a year back.
The Greyhawk book (if not a True First Print) can get anywhere from $10 to $35, or even as much as $50, depending on condition and Printing number.