If you try your best you'll find a poster that is pretty valuable, let's say up to a $1000 which may even be a little attractive.
That is quite a generalization Wim... you clearly aren't trying very hard:
Breakfast at Tiffany's 1-sheet -- $305
In a Lonely Place French 1-panel -- $550
Lot of 18 Star Wars trilogy posters -- $50
Schindler's List Saul Bass 1-sheet -- $211
Schindler's List Saul Bass 1-sheet -- $350
Don't Bother to Knock 1-sheet -- $350
Niagara 1-sheet -- $150
(2) King Kong heralds - $35
I'm sure I'm missing plenty more...
And I'm not even counting my Magnum Force 40x60 I got for $125 as that was from a garage sale.
All of the above were won at auction. If they had been discussed here, do you really think I would have won them for those amounts?