You'll find some 70's D-28 prices among this group, they tend to be a bit all over the place, depends on condition:
http://www.gbase.com/stores/gear/gea...rd=martin+d-28
In addition to other issues, guitars from that era typically need or will need fairly soon a neck reset and compression fret job if they haven't had one, and may need the neck straightened to begin with.
While some of these 70's 28s had/have issues (saddle misplacements, larger "tone killer" bridge plates, over-built bracing), some of them are real gems.
I negotiated a great price on a 1976 D-18 because it had some of these problems, but I also realized it had a great warm, aged tone regardless. I invested in selected repairs and couldn't be happier with the outcome: it sounds better than any current or late model 18 I've tried or heard.
Let the luthier set the price, you shouldn't have to. Stay as neutral as you can, and warn you're buddy he's technically dealing with damaged goods because of the modifications to the soundhole, even though it may sound great. Good Luck!