I have a receipt from a PO for $350 for new headliner and package shelf in my E34. That might give you an idea of what you would pay for headliner. I do have a little different roof since I have a glass sunroof but either way you have to upholster the underpanel.
I have seen a LOT of interiors over the years on ebay. People selling all the door panels, seats and console. So that stuff is out there.
I don't know where you live but in San Diego there is a place called Autobahn that breaks a dozen cars a week probably. They have everything but it might not be cheap enough for you. They have a web site: search Autobahn, El Cajon, BMW and it should pop up.
Mike Holbrook
Meridian, Idaho
1992 535im, 17", Euro M5 Throwing Stars 8's & 9's, FK-451 235/45s & 255/40s, M5 Sway Bars 25/20, Conforti chip, Lowes Ram Air, glass sunroof