Whatever somebody is willing to pay. That offer of $50 for the shell sounds pretty decent. I usually "retire" old helmets by making hanging flower baskets out of them... drill a few holes, add wire.
Might have to give that rebuilder a shot, I have a USA made flat black motorcycle police helmet that needs a new lining, it's only a 1997 model, far too new to put on the shelf or turn into a flower pot. And my Snell Foundation approved (1968) Made in Japan Buco replica needs new fabric too... Come to think of it, so does the 1970s Bell maroon and gold 3/4 shell...
My 2004 HJC police style (model CL-21) needs the styrofoam glued back in place... Can do that at home, piece of cake.