I would have no idea what it would be worth but you could find out by looking on eBay and other auction sites or inquire at a place that buys/sells such kinds of things. I imagine its value will depend on the quality of your copy, how many copies were made or are estimated to still exist, how much interest there is in the particular subject matter or the particular artist, etc.
MATTHEW'S ISLAND 
I own an original of this poster, framed.
Any idea what it is worth?
I would have no idea what it would be worth but you could find out by looking on eBay and other auction sites or inquire at a place that buys/sells such kinds of things. I imagine its value will depend on the quality of your copy, how many copies were made or are estimated to still exist, how much interest there is in the particular subject matter or the particular artist, etc.