For me, if something is above 100 it gets no additional value, since craft numbers above, but not including 100 don't appear in game.

I consider them mostly a game of who's left holding the bag....I see people try to trade low craft number craft hat items for key items, its a way of saying your 1.33 ref digit divulger or whatever is worth 12 keys because you have a craft number under 100, so you try to trade it for a bills and see if they want to be left with something they have to trick someone else into taking;)

Some people actually like certain craft numbers so if you can get one cheap, you can sell it high to the right person and make a nice profit.