"I don't believe Harker was the first to discover this."

As Vic Gammon stated in his 1986 review of 'Fakesong': "There is a sense in which Harker's cut-off from the folk revival gives him
a very odd and undifferentiated view of the movement. What is most
significant is that many of the criticisms which Harker produces in a theoretical mode have been current within the revival for years..."

I don't see that thinking about the material collected and pondering over its nature and origins, and sometimes getting some money for something related to it it, is inconsistant with doing it because it's interesting. Did Sharp cover his costs?

In the USA, which is the period I know most about, Sharp's fieldwork expenses were largely underwritten by his benefactor Helen Storrow in Boston. He made his income (over-estimated by Harker in his letter to FMJ as described earlier) through lecturing and consultancy work. I can't tell you how much money he made from his publications - maybe someone out there can help? I agree with your general point.