Nadjas backed e104-1,2, and 3s are incredibly hard to find. I never had a problem paying extra if it had a nadja back because they rarely ever come up! I think these sets would be more widely traded if they were sold more often; the fact that they don't come around much makes them less popular.
Leon is pretty spot on with the 20-1 blank backed to nadja back ratio, maybe even a tad higher.
Here is an example I have from each subset -
mickey1.jpg
krause1.jpg
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