There's a lot of sites selling Duncans, just as with any coral it all depends on coloration and actual source that will determine the actual price.
Some of the basic, or light green, cream colored Duncans can be picked up for $15-25 per polyp.
ORA Duncans which are neon green can cost $50-$80 per polyp.
Point is, if you find some Duncans for $50, it may or may not be a good deal, the brighter neon green the better.