Denmarks finest (sorry Suz) shows that original modern posters are fine and affordable.
For me, he will always be associated with the 70s but its a tribute to his longevity that he was still producing stunning works well into the noughties
Because of the popularity of the artist, there are lots of original items out there at surprisingly affordable prices. My conclusion : unless they are limited editions, modern posters will always struggle to match the values of their predecessors.