The Norfolk commemorative was one of the many "contrived" commemorative issues of the 1930s. It was supposedly struck to commemorate the 200th anniversary of Norfolk's change from village to full "borough" status in 1736?!?!?!? Contrived though it may have been, 25,013 were distributed at $1.50 per coin ($1.65 by mail). Today, Norfolks are of course a legitimate part of the 50 piece 1892-1954 silver commemorative type set and they have been widely collected since they were issued in 1936.
The interesting thing about Norfolk half dollars is that they are the highest quality commemorative in terms of condition. In fact, the average grade for Norfolks is Gem MS65 to Superb Gem MS67. And MS68s aren't exactly rare either. This is probably due to the fact that the design is so busy. There is so much going on with the design that there isn't a lot of flat field space and consequently Norfolks don't seem to ever have many marks. They also usually have great frosty luster and the strike is always full.
Overall, Norfolks are about as rare as the Elgin, Roanoke, Wisconsin, and York, but in terms of condition rarity, the Norfolk is by far the most common 1892-1954 silver commem in MS67 and MS68 condition. There is, however, one thing about Norfolks that is rare. If you are a "low ball" collector, you will really struggle to find a circulated grade Norfolk. In fact, I have never seen one that graded below AU55.